MySQL Reference Manual for version 3.22.26a.


1 General Information about MySQL

This is the MySQL reference manual; it documents MySQL version 3.22.26a.

MySQL is a very fast, multi-threaded, multi-user and robust SQL (Structured Query Language) database server.

For Unix and OS/2 platforms, MySQL is basically free; for Microsoft platforms you must get a MySQL license after a trial time of 30 days. See section 3 MySQL licensing and support.

The MySQL home page provides the latest information about MySQL.

For a discussion of MySQL's capabilities, see section 1.4 The main features of MySQL.

For installation instructions, see section 4 Installing MySQL. For tips on porting MySQL to new machines or operating systems, see section G Comments on porting to other systems.

For information about upgrading from a 3.21 release, see section 4.16.2 Upgrading from a 3.21 version to 3.22.

For a tutorial introduction to MySQL, see section 8 MySQL Tutorial.

For examples of SQL and benchmarking information, see the benchmarking directory. For source distributions, this is the `bench' directory. For binary distributions, this is the `sql-bench' directory.

For a history of new features and bug fixes, see section D MySQL change history.

For a list of currently known bugs and misfeatures, see section E Known errors and design deficiencies in MySQL.

For future plans, see section F List of things we want to add to MySQL in the future (The TODO).

For a list of all the contributors to this product, see section C Contributors to MySQL.

IMPORTANT:

Send bug (error) reports, questions and comments to the mailing list at

For source distributions, the mysqlbug script can be found in the `scripts' directory. For binary distributions, mysqlbug can be found in the `bin' directory.

If you have any suggestions concerning additions or corrections to this manual, please send them to the MySQL mailing list (mysql@lists.mysql.com) with the following subject line: documentation suggestion: [Insert Topic Here]. See section 2.1 The MySQL mailing lists.

1.1 What is MySQL?

MySQL is a true multi-user, multi-threaded SQL database server. SQL is the most popular database language in the world. MySQL is a client/server implementation that consists of a server daemon mysqld and many different client programs and libraries.

SQL is a standardized language that makes it easy to store, update and access information. For example, you can use SQL to retrieve product information and store customer information for a web site. MySQL is also fast and flexible enough to allow you to store logs and pictures in it.

The main goals of MySQL are speed, robustness and ease of use. MySQL was originally developed because we at TcX needed a SQL server that could handle very large databases an order of magnitude faster than what any database vendor could offer to us. We have now been using MySQL since 1996 in an environment with more than 40 databases containing 10,000 tables, of which more than 500 have more than 7 million rows. This is about 100 gigabytes of mission-critical data.

The base upon which MySQL is built is a set of routines that have been used in a highly demanding production environment for many years. Although MySQL is still under development, it already offers a rich and highly useful function set.

The official way to pronounce MySQL is "My Ess Que Ell" (Not MY-SEQUEL).

1.2 About this manual

This manual is currently available in Texinfo, plain text, Info, HTML, PostScript and PDF versions. Because of their size, PostScript and PDF versions are not included with the main MySQL distribution, but are available for separate download at http://www.mysql.com.

The primary document is the Texinfo file. The HTML version is produced automatically with a modified version of texi2html. The plain text and Info versions are produced with makeinfo. The Postscript version is produced using texi2dvi and dvips. The PDF version is produced with the Ghostscript utility ps2pdf.

This manual is written and maintained by David Axmark, Michael (Monty) Widenius, Paul DuBois and Kim Aldale. For other contributors, see section C Contributors to MySQL.

1.2.1 Conventions used in this manual

This manual uses certain typographical conventions:

constant
Constant-width font is used for command names and options; SQL statements; database, table and column names; C and Perl code; and environment variables. Example: "To see how mysqladmin works, invoke it with the --help option."
`filename'
Constant-width font with surrounding quotes is used for filenames and pathnames. Example: "The distribution is installed under the `/usr/local/' directory."
`c'
Constant-width font with surrounding quotes is also used to indicate character sequences. Example: "To specify a wildcard, use the `%' character."
italic
Italic font is used for emphasis, like this.
boldface
Boldface font is used for access privilege names (e.g., "do not grant the process privilege lightly") and to convey especially strong emphasis.

When commands are shown that are meant to be executed by a particular program, the program is indicated by the prompt shown with the command. For example, shell> indicates a command that you execute from your login shell, and mysql> indicates a command that you execute from the mysql client:

shell> type a shell command here
mysql> type a mysql command here

Shell commands are shown using Bourne shell syntax. If you are using a csh-style shell, you may need to issue commands slightly differently. For example, the sequence to set an environment variable and run a command looks like this in Bourne shell syntax:

shell> VARNAME=value some_command

For csh, you would execute the sequence like this:

shell> setenv VARNAME value
shell> some_command

Database, table and column names often must be substituted into commands. To indicate that such substitution is necessary, this manual uses db_name, tbl_name and col_name. For example, you might see a statement like this:

mysql> SELECT col_name FROM db_name.tbl_name;

This means that if you were to enter a similar statement, you would supply your own database, table and column names, perhaps like this:

mysql> SELECT author_name FROM biblio_db.author_list;

SQL statements may be written in uppercase or lowercase. When this manual shows a SQL statement, uppercase is used for particular keywords if those keywords are under discussion (to emphasize them) and lowercase is used for the rest of the statement. So you might see the following in a discussion of the SELECT statement:

mysql> SELECT count(*) FROM tbl_name;

On the other hand, in a discussion of the COUNT() function, the statement would be written like this:

mysql> select COUNT(*) from tbl_name;

If no particular emphasis is intended, all keywords are written uniformly in uppercase.

In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses:

DROP TABLE [IF EXISTS] tbl_name

When a syntax element consists of a number of alternatives, the alternatives are separated by vertical bars (`|'). When one member from a set of choices may be chosen, the alternatives are listed within square brackets. When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'):

TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)
{DESCRIBE | DESC} tbl_name {col_name | wild}

1.3 History of MySQL

We once started off with the intention of using mSQL to connect to our tables using our own fast low-level (ISAM) routines. However, after some testing we came to the conclusion that mSQL was not fast enough or flexible enough for our needs. This resulted in a new SQL interface to our database but with almost the same API interface as mSQL. This API was chosen to ease porting of third-party code.

The derivation of the name MySQL is not perfectly clear. Our base directory and a large number of our libraries and tools have had the prefix "my" for well over 10 years. However, Monty's daughter (some years younger) is also named My. So which of the two gave its name to MySQL is still a mystery, even for us.

1.4 The main features of MySQL

The following list describes some of the important characteristics of MySQL:

1.5 How stable is MySQL?

This section addresses the questions, "how stable is MySQL?" and, "can I depend on MySQL in this project?" Here we will try to clarify some issues and to answer some of the more important questions that seem to concern many people. This section has been put together from information gathered from the mailing list (which is very active in reporting bugs).

At TcX, MySQL has worked without any problems in our projects since mid-1996. When MySQL was released to a wider public, we noticed that there were some pieces of "untested code" that were quickly found by the new users who made queries in a manner different than our own. Each new release has had fewer portability problems than the previous one (even though each has had many new features), and we hope that it will be possible to label one of the next releases "stable".

Each release of MySQL has been usable and there have been problems only when users start to use code from "the gray zones". Naturally, outside users can't know what the gray zones are; this section attempts to indicate those that are currently known. The descriptions deal with the 3.22.x version of MySQL. All known and reported bugs are fixed in the latest version, with the exception of the bugs listed in the bugs section, which are things that are "design"-related. See section E Known errors and design deficiencies in MySQL.

MySQL is written in multiple layers and different independent modules. These modules are listed below with an indication of how well-tested each of them is:

The ISAM table handler -- Stable
This manages storage and retrieval of all data in MySQL 3.22 and earlier versions. In all MySQL releases there hasn't been a single (reported) bug in this code. The only known way to get a corrupted table is to kill the server in the middle of an update. Even that is unlikely to destroy any data beyond rescue, because all data are flushed to disk between each query. There hasn't been a single bug report about lost data because of bugs in MySQL, either.
The MyISAM table handler -- Beta
This is new in MySQL 3.23. It's largely based on the ISAM table code but has a lot of new very useful features.
The parser and lexical analyser -- Stable
There hasn't been a single reported bug in this system for a long time.
The C client code -- Stable
No known problems. In early 3.20 releases, there were some limitations in the send/receive buffer size. As of 3.21.x, the buffer size is now dynamic up to a default of 24M.
Standard client programs -- Stable
These include mysql, mysqladmin and mysqlshow, mysqldump, and mysqlimport.
Basic SQL -- Stable
The basic SQL function system and string classes and dynamic memory handling. Not a single reported bug in this system.
Query optimizer -- Stable
Range optimizer -- Gamma
Join optimizer -- Stable
Locking -- Gamma
This is very system-dependent. On some systems there are big problems using standard OS locking (fcntl()). In these cases, you should run the MySQL daemon with the --skip-locking flag. Problems are known to occur on some Linux systems and on SunOS when using NFS-mounted file systems.
Linux threads -- Gamma
The only problem found has been with the fcntl() call, which is fixed by using the --skip-locking option to mysqld. Some people have reported lockup problems with the 0.5 release.
Solaris 2.5+ pthreads -- Stable
We use this for all our production work.
MIT-pthreads (Other systems) -- Gamma
There have been no reported bugs since 3.20.15 and no known bugs since 3.20.16. On some systems, there is a "misfeature" where some operations are quite slow (a 1/20 second sleep is done between each query). Of course, MIT-pthreads may slow down everything a bit, but index-based SELECT statements are usually done in one time frame so there shouldn't be a mutex locking/thread juggling.
Other thread implementions -- Alpha - Beta
The ports to other systems are still very new and may have bugs, possibly in MySQL, but most often in the thread implementation itself.
LOAD DATA ..., INSERT ... SELECT -- Stable
Some people have thought they have found bugs here, but these usually have turned out to be misunderstandings. Please check the manual before reporting problems!
ALTER TABLE -- Stable
Small changes in 3.22.12.
DBD -- Stable
Now maintained by Jochen Wiedmann
mysqlaccess -- Stable
Written and maintained by Yves Carlier
GRANT -- Gamma
Big changes made in MySQL 3.22.12.
MyODBC (uses ODBC SDK 2.5) -- Beta
It seems to work well with some programs.

TcX provides email support for paying customers, but the MySQL mailing list usually provides answers to common questions. Bugs are usually fixed right away with a patch; for serious bugs, there is almost always a new release.

1.6 Year 2000 compliance

MySQL itself has no problems with Year 2000 (Y2K) compliance:

You may run into problems with applications that use MySQL in a way that is not Y2K-safe. For example, many old applications store or manipulate years using 2-digit values (which are ambiguous) rather than 4-digit values. This problem may be compounded by applications that use values such as 00 or 99 as "missing" value indicators.

Unfortunately, these problems may be difficult to fix, since different applications may be written by different programmers, each of whom may use a different set of conventions and date-handling functions.

Here is a simple demonstration illustrating that MySQL doesn't have any problems with dates until the year 2030!

mysql> DROP TABLE IF EXISTS y2k;
mysql> CREATE TABLE y2k (date date, date_time datetime, time_stamp timestamp);
mysql> INSERT INTO y2k VALUES ("1998-12-31","1998-12-31 23:59:59",19981231235959);
mysql> INSERT INTO y2k VALUES ("1999-01-01","1999-01-01 00:00:00",19990101000000);
mysql> INSERT INTO y2k VALUES ("1999-09-09","1999-09-09 23:59:59",19990909235959);
mysql> INSERT INTO y2k VALUES ("2000-01-01","2000-01-01 00:00:00",20000101000000);
mysql> INSERT INTO y2k VALUES ("2000-02-28","2000-02-28 00:00:00",20000228000000);
mysql> INSERT INTO y2k VALUES ("2000-02-29","2000-02-29 00:00:00",20000229000000);
mysql> INSERT INTO y2k VALUES ("2000-03-01","2000-03-01 00:00:00",20000301000000);
mysql> INSERT INTO y2k VALUES ("2000-12-31","2000-12-31 23:59:59",20001231235959);
mysql> INSERT INTO y2k VALUES ("2001-01-01","2001-01-01 00:00:00",20010101000000);
mysql> INSERT INTO y2k VALUES ("2004-12-31","2004-12-31 23:59:59",20041231235959);
mysql> INSERT INTO y2k VALUES ("2005-01-01","2005-01-01 00:00:00",20050101000000);
mysql> INSERT INTO y2k VALUES ("2030-01-01","2030-01-01 00:00:00",20300101000000);
mysql> INSERT INTO y2k VALUES ("2050-01-01","2050-01-01 00:00:00",20500101000000);
mysql> SELECT * FROM y2k;
+------------+---------------------+----------------+
| date       | date_time           | time_stamp     |
+------------+---------------------+----------------+
| 1998-12-31 | 1998-12-31 23:59:59 | 19981231235959 |
| 1999-01-01 | 1999-01-01 00:00:00 | 19990101000000 |
| 1999-09-09 | 1999-09-09 23:59:59 | 19990909235959 |
| 2000-01-01 | 2000-01-01 00:00:00 | 20000101000000 |
| 2000-02-28 | 2000-02-28 00:00:00 | 20000228000000 |
| 2000-02-29 | 2000-02-29 00:00:00 | 20000229000000 |
| 2000-03-01 | 2000-03-01 00:00:00 | 20000301000000 |
| 2000-12-31 | 2000-12-31 23:59:59 | 20001231235959 |
| 2001-01-01 | 2001-01-01 00:00:00 | 20010101000000 |
| 2004-12-31 | 2004-12-31 23:59:59 | 20041231235959 |
| 2005-01-01 | 2005-01-01 00:00:00 | 20050101000000 |
| 2030-01-01 | 2030-01-01 00:00:00 | 20300101000000 |
| 2050-01-01 | 2050-01-01 00:00:00 | 00000000000000 |
+------------+---------------------+----------------+

13 rows in set (0.00 sec)
mysql> DROP TABLE y2k;

This shows that the DATE and DATETIME types are Date Data compliant, while the TIMESTAMP type, that is used to store the current time, has a range up to only 2030-01-01. TIMESTAMP has a range of 1970 to 2030 on 32-bit machines.

Even though MySQL is Y2K-compliant, it is your responsibility to provide unambiguous input. See section 7.2.6.1 Y2K issues and date types for MySQL's rules for dealing with ambiguous date input data (data containing 2-digit year values).

1.7 General SQL information and tutorials

This book has been recommended by a several people on the MySQL mailing list:

Judith S. Bowman, Sandra L. Emerson and Marcy Darnovsky
The Practical SQL Handbook: Using Structured Query Language
Second Edition
Addison-Wesley
ISBN 0-201-62623-3
http://www.awl.com

This book has also received some recommendations on the mailing list:

Martin Gruber
Understanding SQL
ISBN 0-89588-644-8
Publisher Sybex 510 523 8233
Alameda, CA USA

A SQL tutorial is available on the net at http://www.geocities.com/SiliconValley/Vista/2207/sql1.html

SQL in 21 Tagen (online book in German language): http://www.mut.de/leseecke/buecher/sql/inhalt.htm

1.8 Useful MySQL-related links

1.8.1 Applications that support MySQL

1.8.2 SQL Clients

1.8.3 Web development tools that support MySQL

1.8.4 Databse design tools with MySQL support

1.8.5 Web servers with MySQL tools

1.8.6 Examples and source

1.8.7 ODBC related links

1.8.8 API related links

1.8.9 Other MySQL-related links

1.8.10 SQL and database interfaces

1.8.11 General database links

There are also many web pages that use MySQL. See section A Some MySQL users. Send any additions to this list to MySQL logo somewhere (It is okay to have it on a "used tools" page or something similar) to be added.

2 MySQL mailing lists and how to ask questions or report errors (bugs)

2.1 The MySQL mailing lists

To subscribe to the main MySQL mailing list, send a message to the electronic mail address mysql-subscribe@lists.mysql.com.

To unsubscribe from the main MySQL mailing list, send a message to the electronic mail address mysql-unsubscribe@lists.mysql.com.

Only the address to which you send your messages is significant. The subject line and the body of the message are ignored.

If your reply address is not valid, you can specify your address explicitly. Adding a hyphen to the subscribe or unsubscribe command word, followed by your address with the `@' character in your address replaced by a `='. For example, to subscribe john@host.domain, send a message to mysql-subscribe-john=host.domain@lists.mysql.com.

Mail to mysql-subscribe@lists.mysql.com or ezmlm mailing list processor. Information about ezmlm is available at the ezmlm Website.

To post a message to the list itself, send your message to mysql@lists.mysql.com. However, please do not send mail about subscribing or unsubscribing to mysql@lists.mysql.com, since any mail sent to that address is distributed automatically to thousands of other users.

Your local site may have many subscribers to mysql@lists.mysql.com. If so, it may have a local mailing list, so that messages sent from lists.mysql.com to your site are propagated to the local list. In such cases, please contact your system administrator to be added to or dropped from the local MySQL list.

The following MySQL mailing lists exist:

announce
This is for announcement of new versions of MySQL and related programs. This is a low volume list that we think all MySQL users should be on.
mysql
The main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialized lists. If you post to the wrong list, you may not get an answer!
mysql-digest
The mysql list in digest form. That means you get all individual messages, sent as one large mail message once a day.
java
Discussion about MySQL and Java. Mostly about the JDBC drivers.
java-digest
A digest version of the java list.
win32
All things concerning MySQL on Microsoft operating systems such as Windows NT.
win32-digest
A digest version of the win32 list.
myodbc
All things concerning connecting to MySQL with ODBC.
myodbc-digest
A digest version of the myodbc list.
msql-mysql-modules
A list about the Perl support in MySQL.
msql-mysql-modules-digest
A digest version of the msql-mysql-modules list.
developer
A list for people who work on the MySQL code.
developer-digest
A digest version of the developer list.

You subscribe or unsubscribe to all lists in the same way as described above. In your subscribe or unsubscribe message, just put the appropriate mailing list name rather than mysql. For example, to subscribe to or unsubscribe from the myodbc list, send a message to

2.2 Asking questions or reporting bugs

Before posting a bug report or question, please do the following:

If you can't find an answer in the manual or the archives, check with your local MySQL expert. If you still can't find an answer to your question, go ahead and read the next section about how to send mail to

2.3 How to report bugs or problems

Writing a good bug report takes patience, but doing it right the first time saves time for us and for you. This section will help you write your report correctly so that you don't waste your time doing things that may not help us much or at all.

We encourage everyone to use the mysqlbug script to generate a bug report (or a report about any problem), if possible. mysqlbug can be found in the `scripts' directory in the source distribution, or, for a binary distribution, in the `bin' directory under your MySQL installation directory. If you are unable to use mysqlbug, you should still include all the necessary information listed in this section.

The mysqlbug script helps you generate a report by determining much of the following information automatically, but if something important is missing, please include it with your message! Please read this section carefully and make sure that all the information described here is included in your report.

Remember that it is possible to respond to a message containing too much information, but not to one containing too little. Often people omit facts because they think they know the cause of a problem and assume that some details don't matter. A good principle is: if you are in doubt about stating something, state it! It is a thousand times faster and less troublesome to write a couple of lines more in your report than to be forced to ask again and wait for the answer because you didn't include enough information the first time.

The most common errors are that people don't indicate the version number of the MySQL distribution they are using, or don't indicate what platform they have MySQL installed on (including the platform version number). This is highly relevant information and in 99 cases out of 100 the bug report is useless without it! Very often we get questions like "Why doesn't this work for me?" and then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has been fixed already in newer MySQL versions. Sometimes the error is platform dependent; in such cases, it is next to impossible to fix anything without knowing the operating system and the version number of the platform.

Remember also to provide information about your compiler, if it is related to the problem. Often people find bugs in compilers and think the problem is MySQL related. Most compilers are under development all the time and become better version by version, too. To determine whether or not your problem depends on your compiler, we need to know what compiler is used. Note that every compiling problem should be regarded as a bug report and reported accordingly.

It is most helpful when a good description of the problem is included in the bug report. That is, a good example of all the things you did that led to the problem and the problem itself exactly described. The best reports are those that include a full example showing how to reproduce the bug or problem.

If a program produces an error message, it is very important to include the message in your report! If we try to search for something from the archives using programs, it is better that the error message reported exactly matches the one that the program produces. (Even the case sensitivity should be observed!) You should never try to remember what the error message was; instead, copy and paste the entire message into your report!

Please include the following information in your report:

If you are a support customer, please cross-post the bug report to the appropriate mailing list to see if someone else has experienced (and perhaps solved) the problem.

For information on reporting bugs in MyODBC, see section 16.2 How to report problems with MyODBC.

For solutions to some common problems, see See section 18 Problems and common errors.

When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarize the answers and send the summary to the mailing list so that others may have the benefit of responses you received that helped you solve your problem!

2.4 Guidelines for answering questions on the mailing list

If you consider your answer to have broad interest, you may want to post it to the mailing list instead of replying directly to the individual who asked. Try to make your answer general enough that people other than the original poster may benefit from it. When you post to the list, please make sure that your answer is not a duplication of a previous answer.

Try to summarize the essential part of the question in your reply; don't feel obliged to quote the entire original message.

Please don't post mail messages from your browser with HTML mode turned on! Many users doesn't read mail with a browser!

3 MySQL licensing and support

This chapter describes MySQL licensing and support arrangements, including:

3.1 MySQL licensing policy

The formal terms of the license for non-Microsoft operating systems such as Unix or OS/2 are specified in section J The MySQL server license for non Microsoft operating systems. Basically, our licensing policy is as follows:

For use under Microsoft operating systems (Win95/Win98/WinNT), you need a MySQL license after a trial period of 30 days, with the exception that licenses may be obtained upon request at no cost for educational use or for university- or government-sponsored research settings. See section K The MySQL license for Microsoft operating systems. A shareware version of MySQL-Win32 that you can try before buying is available at http://www.mysql.com/mysql_w32.htmy. After you have paid, you will get a password that will enable you to access the newest MySQL-Win32 version.

If you have any questions as to whether or not a license is required for your particular use of MySQL, please contact us. See section 3.5.2 Contact information.

If you require a MySQL license, the easiest way to pay for it is to use the license form at TcX's secure server at https://www.mysql.com/license.htmy. Other forms of payment are discussed in section 3.5.1 Payment information.

3.2 Copyrights used by MySQL

There are several different copyrights on the MySQL distribution:

  1. The MySQL-specific source needed to build the mysqlclient library and programs in the `client' directory is in the public domain. Each file that is in the public domain has a header which clearly states so. This includes everything in the `client' directory and some parts of the mysys, mystring and dbug libraries.
  2. Some small parts of the source (GNU getopt) are covered by the "GNU LIBRARY LIBRARY GENERAL PUBLIC LICENSE". See the `mysys/COPYING.LIB' file.
  3. Some small parts of the source (GNU readline) are covered by the "GNU GENERAL PUBLIC LICENSE". See the `readline/COPYING' file.
  4. Some parts of the source (the regexp library) are covered by a Berkeley style copyright.
  5. The other source needed for the MySQL server on non-Microsoft platforms is covered by the "MySQL FREE PUBLIC LICENSE", which is based on the "Aladdin FREE PUBLIC LICENSE." See section J The MySQL server license for non Microsoft operating systems. When running MySQL on any Microsoft operating system, other licensing applies.

The following points set forth the philosophy behind our copyright policy:

3.2.1 Possible future copyright changes

We may choose to distribute older versions of MySQL with the GPL in the future. However, these versions will be identified as GNU MySQL. Also, all copyright notices in the relevant files will be changed to the GPL.

3.3 Distributing MySQL commercially

This section is a clarification of the license terms that are set forth in the "MySQL FREE PUBLIC LICENSE" (FPL). See section J The MySQL server license for non Microsoft operating systems.

MySQL may be used freely, including by commercial entities for evaluation or unsupported internal use. However, distribution for commercial purposes of MySQL, or anything containing or derived from MySQL in whole or in part, requires a written commercial license from TcX AB, the sole entity authorized to grant such licenses.

You may not include MySQL "free" in a package containing anything for which a charge is being made, except as noted below.

The intent of the exception provided in the second clause of the license is to allow commercial organizations operating an FTP server or a bulletin board to distribute MySQL freely from it, provided that:

  1. The organization complies with the other provisions of the FPL, which include among other things a requirement to distribute the full source code of MySQL and of any derived work, and to distribute the FPL itself along with MySQL;
  2. The only charge for downloading MySQL is a charge based on the distribution service and not one based on the content of the information being retrieved (i.e., the charge would be the same for retrieving a random collection of bits of the same size);
  3. The server or BBS is accessible to the general public, i.e., the phone number or IP address is not kept secret, and anyone may obtain access to the information (possibly by paying a subscription or access fee that is not dependent on or related to purchasing anything else).

If you want to distribute software in a commercial context that incorporates MySQL and you do not want to meet these conditions, you should contact TcX AB to find out about commercial licensing, which involves a payment. The only ways you legally can distribute MySQL or anything containing MySQL are by distributing MySQL under the requirements of the FPL, or by getting a commercial license from TcX AB.

3.4 Example licensing situations

This section describes some situations illustrating whether or not you must license the MySQL server. Generally these examples involve providing MySQL as part of a product or service that you are selling to a customer, or requiring that MySQL be used in conjunction with your product. In such cases, it is your responsibility to obtain a license for the customer if one is necessary. (This requirement is waived if your customer already has a MySQL license. But the seller must send customer information and the license number to TcX, and the license must be a full license, not an OEM license.)

Note that a single MySQL license covers any number of CPUs/users/customers/mysqld servers on a machine!

3.4.1 Selling products that use MySQL

To determine whether or not you need a MySQL license when selling your application, you should ask whether the proper functioning of your application is contingent on the use of MySQL and whether you include MySQL with your product. There are several cases to consider:

3.4.2 Selling MySQL-related services

If you perform MySQL installation on a client's machine and any money changes hands for the service (directly or indirectly), then you must buy a MySQL license.

If you sell an application for which MySQL is not strictly required but can be used, a license may be indicated, depending on how MySQL is set up. Suppose your product neither requires MySQL nor includes it in your product distribution, but can be configured to use MySQL for those customers who so desire. (This would be the case, for example, if your product can use any of a number of database engines.)

If the customer obtains and installs MySQL, no license is needed. If you perform that service for your customer, then a license is needed because then you are selling a service that includes MySQL.

3.4.3 ISP MySQL services

Internet Service Providers (ISPs) often host MySQL servers for their customers.

If you are an ISP that allows customers to install and administer MySQL for themselves on your machine with no assistance from you, neither you nor your customer need a MySQL license.

If you charge for MySQL installation and administrative support as part of your customer service, then you need a license because you are selling a service that includes MySQL.

3.4.4 Running a web server using MySQL

If you use MySQL in conjunction with a web server, you don't have to pay for a license.

This is true even if you run a commercial web server that uses MySQL, since you are not selling MySQL itself. However, in this case we would like you to purchase MySQL support, because MySQL is helping your enterprise.

3.5 MySQL licensing and support costs

Our current license prices are shown below. All prices are in US Dollars. If you pay by credit card, the currency is EURO (European Union Euro) so the prices will differ slightly.

Number of licenses Price per copy Total
1 US $200 US $200
10 pack US $150 US $1500
50 pack US $120 US $6000

For high volume (OEM) purchases, the following prices apply:

Number of licenses Price per copy Minimum at one time Minimum payment
100-999 US $40 100 US $4000
1000-2499 US $25 200 US $5000
2500-4999 US $20 400 US $8000

For OEM purchases, you must act as the middle-man for eventual problems or extension requests from your users. We also require that OEM customers have at least an extended email support contract.

If you have a low-margin high-volume product, you can always talk to us about other terms (for example, a percent of the sale price). If you do, please be informative about your product, pricing, market and any other information that may be relevant.

After buying 10 MySQL licenses, you will get a personal copy of the pack_isam utility. You are not allowed to redistribute this utility but you can distribute tables packed with it.

A full-price license is not a support agreement and includes very minimal support. This means that we try to answer any relevant question. If the answer is in the documentation, we will direct you to the appropriate section. If you have not purchased a license or support, we probably will not answer at all.

If you discover what we consider a real bug, we are likely to fix it in any case. But if you pay for support we will notify you about the fix status instead of just fixing it in a later release.

More comprehensive support is sold separately. Descriptions of what each level of support includes are given in section 3.6 Types of commercial support. Costs for the various types of commercial support are shown below. Support level prices are in EURO (European Union Euro). One EURO is about 1.17 USD.

Type of support Cost per year
Basic email support EURO 170
Extended email support EURO 1000
Login support EURO 2000
Extended login support EURO 5000

You may upgrade from any lower level of support to a higher level of support for the difference between the prices of the two support levels.

3.5.1 Payment information

Currently we can take SWIFT payments, cheques or credit cards.

Payment should be made to:

Postgirot Bank AB
105 06 STOCKHOLM, SWEDEN

TCX DataKonsult AB
BOX 6434
11382 STOCKHOLM, SWEDEN

SWIFT address: PGSI SESS
Account number: 96 77 06 - 3

Specify: license and/or support and your name and email address.

In Europe and Japan you can use EuroGiro (that should be less expensive) to the same account.

If you want to pay by cheque, make it payable to "Monty Program KB" and mail it to the address below:

TCX DataKonsult AB
BOX 6434
11382 STOCKHOLM, SWEDEN

If you want to pay by credit card over the Internet, you can use TcX's secure license form.

You can also print a copy of the license form, fill it in and send it by fax to:

+46-8-729 69 05

If you want us to bill you, you can use the license form and write "bill us" in the comment field. You can also mail a message to with your company information and ask us to bill you.

3.5.2 Contact information

For commercial licensing, or if you have any questions about any of the information in this section, please contact the MySQL licensing team. The much preferred method is by E-Mail to these may take much longer (Fax +46-8-729 69 05).

David Axmark
Detron HB
Kungsgatan 65 B
753 21 UPPSALA
SWEDEN
Voice Phone +46-18-10 22 80     (Timezone GMT+1. Swedish and English spoken)

3.6 Types of commercial support

3.6.1 Basic email support

Basic email support is a very inexpensive support option and should be thought of more as a way to support our development of MySQL than as a real support option.

At this support level, the MySQL mailing lists are the preferred means of communication. Questions normally should be mailed to the primary mailing list (mysql@lists.mysql.com) or one of the other regular lists (for example, mysql-win32@lists.mysql.com for Windows-related MySQL questions), as someone else already may have experienced and solved the problem you have. See section 2.2 Asking questions or reporting bugs.

However, by purchasing basic email support, you also have access to the support address mysql-support@mysql.com, which is not available as part of the minimal support that you get by purchasing a MySQL license. This means that for especially critical questions, you can cross-post your message to mysql-support@mysql.com. (If the message contains sensitive data, you should post only to mysql-support@mysql.com.)

REMEMBER! to ALWAYS include your registration number and expiration date when you send a message to

Basic email support includes the following types of service:

3.6.2 Extended email support

Extended email support includes everything in basic email support with these additions:

3.6.3 Login support

Login support includes everything in extended email support with these additions:

3.6.4 Extended login support

Extended login support includes everything in login support with these additions:

4 Installing MySQL

This chapter describes how to obtain and install MySQL:

4.1 How to get MySQL

Check the MySQL home page for information about the current version and for downloading instructions.

However, the Internet connection at TcX is not so fast; we would prefer that you do the actual downloading from one of the mirror sites listed below.

Please report bad or out of date mirrors to webmaster@mysql.com.

Europe:

North America:

South America:

Asia:

Australia:

Africa:

4.2 Operating systems supported by MySQL

We use GNU Autoconf so it is possible to port MySQL to all modern systems with working Posix threads and a C++ compiler. (To compile only the client code, a C++ compiler is required but not threads.) We use and develop the software ourselves primarily on Sun Solaris (versions 2.5 & 2.6) and to a lesser extent on RedHat Linux 5.0.

MySQL has been reported to compile sucessfully on the following operating system/thread package combinations. Note that for many operating systems, the native thread support works only in the latest versions.

4.3 Which MySQL version to use

The first decision to make is whether you want to use the latest development release or the last stable release:

The second decision to make is whether you want to use a source distribution or a binary distribution:

The MySQL naming scheme uses release numbers that consist of three numbers and a suffix. For example, a release name like mysql-3.21.17-beta is interpreted like this:

All versions of MySQL are run through our standard tests and benchmarks to ensure that they are relatively safe to use. Since the standard tests are extended over time to check for all previously found bugs, the test suite keeps getting better.

Note that all releases have been tested at least with:

An internal test suite
This is part of a production system for a customer. It has many tables with hundreds of megabytes of data.
The MySQL benchmark suite
This runs a range of common queries. It is also a test to see whether the latest batch of optimizations actually made the code faster. See section 11 The MySQL benchmark suite.
The crash-me test
This tries to determine what features the database supports and what its capabilities and limitations are. See section 11 The MySQL benchmark suite.

Another test is that we use the newest MySQL version in our internal production environment, on at least one machine. We have more than 100 gigabytes of data to work with.

4.4 How and when updates are released

MySQL is evolving quite rapidly here at TcX and we want to share this with other MySQL users. We try to make a release when we have very useful features that others seem to have a need for.

We also try to help out users who request features that are easy to implement. We also take note of what our licensed users want to have and we especially take note of what our extended email supported customers want and try to help them out.

No one has to download a new release. The News section will tell you if the new release has something you really want. See section D MySQL change history.

We use the following policy when updating MySQL:

The current stable release is 3.22; We have already moved active development to 3.23. Bugs will still be fixed in the stable version. We don't believe in a complete freeze, as this also leaves out bug fixes and things that "must be done". "Somewhat frozen" means that we may add small things that "almost surely will not affect anything that's already working".

4.5 Installation layouts

This section describes the default layout of the directories created by installing binary and source distributions.

A binary distribution is installed by unpacking it at the installation location you choose (typically `/usr/local/mysql') and creates the following directories in that location:

Directory Contents of directory
`bin' Client programs and the mysqld server
`data' Log files, databases
`include' Include (header) files
`lib' Libraries
`scripts' mysql_install_db
`share/mysql' Error message files
`sql-bench' Benchmarks

A source distribution is installed after you configure and compile it. By default, the installation step installs files under `/usr/local', in the following subdirectories:

Directory Contents of directory
`bin' Client programs and scripts
`include/mysql' Include (header) files
`info' Documentation in Info format
`lib/mysql' Libraries
`libexec' The mysqld server
`share/mysql' Error message files
`sql-bench' Benchmarks and crash-me test
`var' Databases and log files.

Within an installation directory, the layout of a source installation differs from that of a binary installation in the following ways:

4.6 Installing a MySQL binary distribution

You need the following tools to install a MySQL binary distribution:

An alternative installation method under Linux is to use RPM (RedHat Package Manager) distributions. See section 4.6.1 Linux RPM notes.

If you run into problems, PLEASE ALWAYS USE mysqlbug when posting questions to mysql@lists.mysql.com. Even if the problem isn't a bug, mysqlbug gathers system information that will help others solve your problem. By not using mysqlbug, you lessen the likelihood of getting a solution to your problem! You will find mysqlbug in the `bin' directory after you unpack the distribution. See section 2.3 How to report bugs or problems.

The basic commands you must execute to install and use a MySQL binary distribution are:

shell> gunzip < mysql-VERSION-OS.tar.gz | tar xvf -
shell> ln -s mysql-VERSION-OS mysql
shell> cd mysql
shell> scripts/mysql_install_db
shell> bin/safe_mysqld &

You can add new users using the bin/mysql_setpermission script if you install the DBI and Msql-Mysql-modules Perl modules.

Here follows a more detailed description:

To install a binary distribution, follow the steps below, then proceed to section 4.15 Post-installation setup and testing, for post-installation setup and testing:

  1. Pick the directory under which you want to unpack the distribution, and move into it. In the example below, we unpack the distribution under `/usr/local' and create a directory `/usr/local/mysql' into which MySQL is installed. (The following instructions therefore assume you have permission to create files in `/usr/local'. If that directory is protected, you will need to perform the installation as root.)
  2. Obtain a distribution file from one of the sites listed in section 4.1 How to get MySQL. MySQL binary distributions are provided as compressed tar archives and have names like `mysql-VERSION-OS.tar.gz', where VERSION is a number (e.g., 3.21.15), and OS indicates the type of operating system for which the distribution is intended (e.g., pc-linux-gnu-i586).
  3. Unpack the distribution and create the installation directory:
    shell> gunzip < mysql-VERSION-OS.tar.gz | tar xvf -
    shell> ln -s mysql-VERSION-OS mysql
    
    The first command creates a directory named `mysql-VERSION-OS'. The second command makes a symbolic link to that directory. This lets you refer more easily to the installation directory as `/usr/local/mysql'.
  4. Change into the installation directory:
    shell> cd mysql
    
    You will find several files and subdirectories in the mysql directory. The most important for installation purposes are the `bin' and `scripts' subdirectories.
    `bin'
    This directory contains client programs and the server You should add the full pathname of this directory to your PATH environment variable so that your shell finds the MySQL programs properly.
    `scripts'
    This directory contains the mysql_install_db script used to initialize the server access permissions.
  5. If you would like to use mysqlaccess and have the MySQL distribution in some nonstandard place, you must change the location where mysqlaccess expects to find the mysql client. Edit the `bin/mysqlaccess' script at approximately line 18. Search for a line that looks like this:
    $MYSQL     = '/usr/local/bin/mysql';    # path to mysql executable
    
    Change the path to reflect the location where mysql actually is stored on your system. If you do not do this, you will get a broken pipe error when you run mysqlaccess.
  6. Create the MySQL grant tables (necessary only if you haven't installed MySQL before):
    shell> scripts/mysql_install_db
    
  7. If you want to install support for the Perl DBI/DBD interface, see section 4.10 Perl installation comments.
  8. If you would like MySQL to start automatically when you boot your machine, you can copy support-files/mysql.server to the location where your system has its startup files. More information can be found in the support-files/mysql.server script itself, and in section 4.15.3 Starting and stopping MySQL automatically.

After everything has been unpacked and installed, you should initialize and test your distribution.

You can start the MySQL server with the following command:

shell> bin/safe_mysqld &

Note that MySQL versions older than 3.22.10 started the MySQL server when you run mysql_install_db. This is no longer true!

See section 4.15 Post-installation setup and testing.

4.6.1 Linux RPM notes

The recommended way to install MySQL on Linux is by using an RPM file. The MySQL RPMs are currently being built on a RedHat 5.2 system but should work on other versions of Linux that support rpm and use glibc.

If you have problems with an RPM file, for example Sorry, the host 'xxxx' could not be looked up, see section 4.6.3.1 Linux notes.

The RPM files you may want to use are:

To see all files in an RPM package:

shell> rpm -qpl MySQL-VERSION.i386.rpm

To perform a standard minimal installation, run this command:

shell> rpm -i MySQL-VERSION.i386.rpm MySQL-client-VERSION.i386.rpm

To install just the client package:

shell> rpm -i MySQL-client-VERSION.i386.rpm

The RPM places data in `/var/lib/mysql'. The RPM also creates the appropriate entries in `/sbin/rc.d/' to start the server automatically at boot time. (This means that if you have performed a previous installation, you may want to make a copy of your previously-installed MySQL startup file if you made any changes to it, so you don't lose your changes.)

After installing the RPM file(s), go to the binary install section and use the instructions there, starting from the step that creates the MySQL grant tables. See section 4.6 Installing a MySQL binary distribution.

4.6.2 Building client programs

If you compile MySQL clients that you've written yourself or that you obtain from a third party, they must be linked using the -lmysqlclient option on the link command. You may also need to specify a -L option to tell the linker where to find the library. For example, if the library is installed in `/usr/local/mysql/lib', use -L/usr/local/mysql/lib -lmysqlclient on the link command.

For clients that use MySQL header files, you may need to specify a -I option when you compile them (for example, -I/usr/local/mysql/include), so the compiler can find the header files.

4.6.3 System-specific issues

The following sections indicate some of the issues that have been observed to occur on particular systems when installing MySQL from a binary distribution.

4.6.3.1 Linux notes

MySQL needs at least Linux 2.0.

The binary release is linked with -static, which means you not normally need not worry about which version of the system libraries you have. You need not install LinuxThreads, either. A program linked with -static is slightly bigger than a dynamically-linked program but also slightly faster (3-5%). One problem however is that you can't use user definable functions (UDFs) with a statically-linked program. If you are going to write or use UDF functions (this is something only for C or C++ programmers) you must compile MySQL yourself, using dynamic linking.

If you are using a libc-based system (instead of a glibc2 system), you will probably get some problems with hostname resolving and getpwnam() with the binary release. (This is because glibc unfortunately depends on some external libraries to resolve hostnames and getwpent() , even when compiled with -static). In this case you probably get the following error message when you run mysql_install_db:

Sorry, the host 'xxxx' could not be looked up

or the following error when you try to run mysqld with the --user option:

getpwnam: No such file or directory

You can solve this problem one of the following ways:

The Linux-Intel binary and RPM releases of MySQL are configured for the highest possible speed. We are always trying to use the fastest stable compiler available.

MySQL Perl support requires Perl 5.004_03 or newer.

4.6.3.2 HP-UX notes

The binary distribution of MySQL for HP-UX is distributed as an HP depot file and as a tar file. To use the depot file you must be running at least HP-UX 10.x to have access to HP's software depot tools.

The HP version of MySQL was compiled on an HP 9000/8xx server under HP-UX 10.20, and uses MIT-pthreads. It is known to work well under this configuration. MySQL 3.22.26 and newer can also be built with HP's native thread package.

Other configurations that may work:

The following configurations almost definitely won't work:

To install the distribution, use one of the commands below, where /path/to/depot is the full pathname of the depot file:

The depot places binaries and libraries in `/opt/mysql' and data in `/var/opt/mysql'. The depot also creates the appropriate entries in `/sbin/init.d' and `/sbin/rc2.d' to start the server automatically at boot time. Obviously, this entails being root to install.

To install the HP-UX tar distribution, you must have a copy of gnu tar.

4.7 Installing a MySQL source distribution

You need the following tools to build and install MySQL from source:

If you run into problems, PLEASE ALWAYS USE mysqlbug when posting questions to mysql@lists.mysql.com. Even if the problem isn't a bug, mysqlbug gathers system information that will help others solve your problem. By not using mysqlbug, you lessen the likelihood of getting a solution to your problem! You will find mysqlbug in the `scripts' directory after you unpack the distribution. See section 2.3 How to report bugs or problems.

4.7.1 Quick installation overview

The basic commands you must execute to install a MySQL source distribution are (from an unpacked tar file):

shell> configure
shell> make
shell> make install
shell> scripts/mysql_install_db
shell> /usr/local/mysql/bin/safe_mysqld &

If you start from a source RPM, then do the following.

shell> rpm --rebuild MySQL-VERSION.src.rpm

This will make a binary RPM that you can install.

You can add new users using the bin/mysql_setpermission script if you install the DBI and Msql-Mysql-modules Perl modules.

Here follows a more detailed description:

To install a source distribution, follow the steps below, then proceed to section 4.15 Post-installation setup and testing, for post-installation initialization and testing.

  1. Pick the directory under which you want to unpack the distribution, and move into it.
  2. Obtain a distribution file from one of the sites listed in section 4.1 How to get MySQL. MySQL source distributions are provided as compressed tar archives and have names like `mysql-VERSION.tar.gz', where VERSION is a number like 3.22.26a.
  3. Unpack the distribution into the current directory:
    shell> gunzip < mysql-VERSION.tar.gz | tar xvf -
    
    This command creates a directory named `mysql-VERSION'.
  4. Change into the top-level directory of the unpacked distribution:
    shell> cd mysql-VERSION
    
  5. Configure the release and compile everything:
    shell> ./configure --prefix=/usr/local/mysql
    shell> make
    
    When you run configure, you might want to specify some options. Run ./configure --help for a list of options. section 4.7.3 Typical configure options, discusses some of the more useful options. If configure fails, and you are going to send mail to lines from `config.log' that you think can help solve the problem. Also include the last couple of lines of output from configure if configure aborts. Post the bug report using the mysqlbug script. See section 2.3 How to report bugs or problems. If the compile fails, see section 4.8 Problems compiling?, for help with a number of common problems.
  6. Install everything:
    shell> make install
    
    You might need to run this command as root.
  7. Create the MySQL grant tables (necessary only if you haven't installed MySQL before):
    shell> scripts/mysql_install_db
    
    Note that MySQL versions older than 3.22.10 started the MySQL server when you run mysql_install_db. This is no longer true!
  8. If you want to install support for the Perl DBI/DBD interface, see section 4.10 Perl installation comments.
  9. If you would like MySQL to start automatically when you boot your machine, you can copy support-files/mysql.server to the location where your system has its startup files. More information can be found in the support-files/mysql.server script itself, and in section 4.15.3 Starting and stopping MySQL automatically.

After everything has been installed, you should initialize and test your distribution.

You can start the MySQL server with the following command, where BINDIR is the directory in which safe_mysqld is installed (`/usr/local/bin' by default):

shell> BINDIR/safe_mysqld &

If that command fails immediately with mysqld daemon ended then you can find some information in the file `mysql-data-directory/'hostname'.err'. The likely reason is that you already have another mysqld server running. See section 19.3 Running multiple MySQL servers on the same machine.

See section 4.15 Post-installation setup and testing.

4.7.2 Applying patches

Sometimes patches appear on the mailing list or are placed in the patches area of the MySQL FTP site.

To apply a patch from the mailing list, save the message in which the patch appears in a file, change into the top-level directory of your MySQL source tree and run these commands:

shell> patch -p1 < patch-file-name
shell> rm config.cache
shell> make clean

Patches from the FTP site are distributed as plain text files or as files compressed with gzip files. Apply a plain patch as shown above for mailing list patches. To apply a compressed patch, change into the top-level directory of your MySQL source tree and run these commands:

shell> gunzip < patch-file-name.gz | patch -p1
shell> rm config.cache
shell> make clean

After applying a patch, follow the instructions for a normal source install, beginning with the ./configure step. After running the make install step, restart your MySQL server.

You may need to bring down any currently running server before you run make install. (Use mysqladmin shutdown to do this.) Some systems do not allow you to install a new version of a program if it replaces the version that is currently executing.

4.7.3 Typical configure options

The configure script gives you a great deal of control over how you configure your MySQL distribution. Typically you do this using options on the configure command line. You can also affect configure using certain environment variables. For a list of options supported by configure, run this command:

shell> ./configure --help

Some of the more commonly-used configure options are described below:

4.8 Problems compiling?

All MySQL programs compile cleanly for us with no warnings on Solaris using gcc. On other systems, warnings may occur due to differences in system include files. See section 4.9 MIT-pthreads notes, for warnings that may occur when using MIT-pthreads. For other problems, check the list below.

The solution to many problems involves reconfiguring. If you do need to reconfigure, take note of the following:

To prevent old configuration information or object files from being used, run these commands before rerunning configure:

shell> rm config.cache
shell> make clean

Alternatively, you can run make distclean.

The list below describes some of the problems compiling MySQL that have been found to occur most often:

4.9 MIT-pthreads notes

This section describes some of the issues involved in using MIT-pthreads.

Note that on Linux you should NOT use MIT-pthreads but install LinuxThreads! See section 4.11.5 Linux notes (all Linux versions).

If your system does not provide native thread support, you will need to build MySQL using the MIT-pthreads package. This includes most FreeBSD systems, SunOS 4.x, Solaris 2.4 and earlier, and some others. See section 4.2 Operating systems supported by MySQL.

4.10 Perl installation comments

4.10.1 Installing Perl on Unix

Perl support for MySQL is provided by means of the DBI/DBD client interface. See section 20.5 MySQL Perl API. The Perl DBD/DBI client code requires Perl 5.004 or later. The interface will not work if you have an older version of Perl.

MySQL Perl support also requires that you've installed MySQL client programming support. If you installed MySQL from RPM files, client programs are in the client RPM, but client programming support is in the developer RPM. Make sure you've installed the latter RPM.

As of release 3.22.8, Perl support is distributed separately from the main MySQL distribution. If you want to install Perl support, the files you will need can be obtained from http://www.mysql.com/Contrib.

The Perl distributions are provided as compressed tar archives and have names like `MODULE-VERSION.tar.gz', where MODULE is the module name and VERSION is the version number. You should get the Data-Dumper, DBI, and Msql-Mysql-modules distributions and install them in that order. The installation procedure is shown below. The example shown is for the Data-Dumper module, but the procedure is the same for all three distributions.

  1. Unpack the distribution into the current directory:
    shell> gunzip < Data-Dumper-VERSION.tar.gz | tar xvf -
    
    This command creates a directory named `Data-Dumper-VERSION'.
  2. Change into the top-level directory of the unpacked distribution:
    shell> cd Data-Dumper-VERSION
    
  3. Build the distribution and compile everything:
    shell> perl Makefile.PL
    shell> make
    shell> make test
    shell> make install
    

The make test command is important, because it verifies that the module is working. Note that when you run that command during the Msql-Mysql-modules installation to exercise the interface code, the MySQL server must be running or the test will fail.

It is a good idea to rebuild and reinstall the Msql-Mysql-modules distribution whenever you install a new release of MySQL, particularly if you notice symptoms such as all your DBI scripts dumping core after you upgrade MySQL.

If you don't have the right to install Perl modules in the system directory or if you to install local Perl modules, the following reference may help you:

http://www.iserver.com/support/contrib/perl5/modules.html

Look under the heading Installing New Modules that Require Locally Installed Modules.

4.10.2 Installing ActiveState Perl on Win32

To install the MySQL DBD module with ActiveState Perl on Win32, you should do the following:

If you can't get the above to work, you should instead install the MyODBC driver and connect to MySQL server through ODBC.

use DBI;
$dbh= DBI->connect("DBI:ODBC:$dsn","$user","$password") || 
  die "Got error $DBI::errstr when connecting to $dsn\n";

4.10.3 Installing the MySQL Perl distribution on Win32

The MySQL Perl distribution contains DBI, DBD:MySQL and DBD:ODBC.

4.10.4 Problems using the Perl DBI/DBD interface

If Perl reports that it can't find the ../mysql/mysql.so module, then the problem is probably that Perl can't locate the shared library `libmysqlclient.so'.

You can fix this by any of the following methods:

If you get the following errors from DBD-mysql, you are probably using gcc (or using an old binary compiled with gcc):

/usr/bin/perl: can't resolve symbol '__moddi3'
/usr/bin/perl: can't resolve symbol '__divdi3'

Add -L/usr/lib/gcc-lib/... -lgcc to the link command when the `mysql.so' library gets built (check the output from make for `mysql.so' when you compile the Perl client). The -L option should specify the pathname of the directory where `libgcc.a' is located on your system.

Another cause of this problem may be that Perl and MySQL aren't both compiled with gcc. In this case, you can solve the mismatch by compiling both with gcc.

If you want to use the Perl module on a system that doesn't support dynamic linking (like SCO) you can generate a static version of Perl that includes DBI and DBD-mysql. The way this works is that you generate a version of Perl with the DBI code linked in and install it on top of your current Perl. Then you use that to build a version of Perl that additionally has the DBD code linked in, and install that.

On SCO, you must have the following environment variables set:

shell> LD_LIBRARY_PATH=/lib:/usr/lib:/usr/local/lib:/usr/progressive/lib
or
shell> LD_LIBRARY_PATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:/usr/progressive/lib:/usr/skunk/lib
shell> LIBPATH=/usr/lib:/lib:/usr/local/lib:/usr/ccs/lib:/usr/progressive/lib:/usr/skunk/lib
shell> MANPATH=scohelp:/usr/man:/usr/local1/man:/usr/local/man:/usr/skunk/man:

First, create a Perl that includes a statically-linked DBI by running these commands in the directory where your DBI distribution is located:

shell> perl Makefile.PL LINKTYPE=static
shell> make
shell> make install
shell> make perl

Then you must install the new Perl. The output of make perl will indicate the exact make command you will need to execute to perform the installation. On SCO, this is make -f Makefile.aperl inst_perl MAP_TARGET=perl.

Next, use the just-created Perl to create another Perl that also includes a statically-linked DBD::mysql by running these commands in the directory where your Msql-Mysql-modules distribution is located:

shell> perl Makefile.PL LINKTYPE=static
shell> make
shell> make install
shell> make perl

Finally, you should install this new Perl. Again, the output of make perl indicates the command to use.

4.11 System-specific issues

The following sections indicate some of the issues that have been observed to occur on particular systems when installing MySQL from a source distribution.

4.11.1 Solaris notes

On Solaris, you may run into trouble even before you get the MySQL distribution unpacked! Solaris tar can't handle long file names, so you may see an error like this when you unpack MySQL:

x mysql-3.22.12-beta/bench/Results/ATIS-mysql_odbc-NT_4.0-cmp-db2,informix,ms-sql,mysql,oracle,solid,sybase, 0 bytes, 0 tape blocks
tar: directory checksum error

In this case, you must use GNU tar (gtar) to unpack the distribution. You can find a precompiled copy for Solaris at http://www.mysql.com/Downloads/.

Sun native threads work only on Solaris 2.5 and higher. For 2.4 and earlier versions, MySQL will automatically use MIT-pthreads. See section 4.9 MIT-pthreads notes.

If you get the following error from configure:

checking for restartable system calls... configure: error can not run test
programs while cross compiling

This means that you have something wrong with your compiler installation! In this case you should upgrade your compiler to a newer version. You may also be able to solve this problem by inserting the following row into the config.cache file:

ac_cv_sys_restartable_syscalls=${ac_cv_sys_restartable_syscalls='no'}

If you are using Solaris on a SPARC, the recommended compiler is egcs 1.1.2 or newer. You can find this at http://egcs.cygnus.com/. Note that egs 1.1.1 and gcc 2.8.1 don't work reliably on SPARC!

The recommended configure line when using egcs 1.1.2 is:

shell> CC=gcc CFLAGS="-O6" \
       CXX=gcc CXXFLAGS="-O6 -felide-constructors -fno-exceptions -fno-rtti" \
       ./configure --prefix=/usr/local/mysql --with-low-memory

If you have the Sun Workshop 4.2 compiler, you can run configure like this:

CC=cc CFLAGS="-xstrconst -Xa -xO4 -native -mt" CXX=CC CXXFLAGS="-xO4 -native -noex -mt" ./configure --prefix=/usr/local/mysql

shell> CC=cc CFLAGS="-Xa -fast -xO4 -native -xstrconst -mt" \
       CXX=CC CXXFLAGS="-noex -XO4 -mt" \
       ./configure

You may also have to edit the configure script to change this line:

#if !defined(__STDC__) || __STDC__ != 1

to this:

#if !defined(__STDC__)

If you turn on __STDC__ with the -Xc option, the Sun compiler can't compile with the Solaris `pthread.h' header file. This is a Sun bug (broken compiler or broken include file).

If mysqld issues the error message shown below when you run it, you have tried to compile MySQL with the Sun compiler without enabling the multi-thread option (-mt):

libc internal error: _rmutex_unlock: rmutex not held

Add -mt to CFLAGS and CXXFLAGS and try again.

If you get the following error when compiling MySQL with gcc, it means that your gcc is not configured for your version of Solaris!

shell> gcc -O3 -g -O2 -DDBUG_OFF  -o thr_alarm ...
./thr_alarm.c: In function `signal_hand':
./thr_alarm.c:556: too many arguments to function `sigwait'

The proper thing to do in this case is to get the newest version of egcs and compile it with your current gcc compiler! At least for Solaris 2.5, almost all binary versions of gcc have old, unusable include files that will break all programs that use threads (and possibly other programs)!

Solaris doesn't provide static versions of all system libraries (libpthreads and libdl), so you can't compile MySQL with --static. If you try to do so, you will get the error:

ld: fatal: library -ldl: not found

If too many processes try to connect very rapidly to mysqld, you will see this error in the MySQL log:

Error in accept: Protocol error

You might try starting the server with the --set-variable back_log=50 option as a workaround for this.

If you are linking your own MySQL client, you might get the following error when you try to execute it:

ld.so.1: ./my: fatal: libmysqlclient.so.#: open failed: No such file or directory

The problem can be avoided by one of the following methods:

4.11.2 Solaris 2.7 notes

You can normally use a Solaris 2.6 binary on Solaris 2.7. Most of the Solaris 2.6 issues also apply for Solaris 2.7.

Solaris 2.7 has some bugs in the include files. You may see the following error when you use gcc:

/usr/include/widec.h:42: warning: `getwc' redefined
/usr/include/wchar.h:326: warning: this is the location of the previous
definition

If this occurs, you can do the following to fix the problem:

Copy /usr/include/widec.h to .../lib/gcc-lib/os/gcc-version/include and change line 41 from:

#if     !defined(lint) && !defined(__lint)

to

#if     !defined(lint) && !defined(__lint) && !defined(getwc)

Alternatively, you can edit `/usr/include/widec.h' directly. Either way, after you make the fix, you should remove `config.cache' and run configure again!

If you get errors like this when you run make, it's because configure didn't detect the `curses.h' file (probably because of the error in /usr/include/widec.h:

In file included from mysql.cc:50:
/usr/include/term.h:1060: syntax error before `,'
/usr/include/term.h:1081: syntax error before `;'

The solution to this is to do one of the following steps:

4.11.3 Solaris x86 notes

If you are using gcc or egcs on Solaris x86 and you experience problems with core dumps under load, you should use the following configure command:

shell> CC=gcc CFLAGS="-O6 -fomit-frame-pointer" \
       CXX=gcc \
       CXXFLAGS="-O6 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" \
       ./configure --prefix=/usr/local/mysql

This will avoid problems with the libstdc++ library and with C++ exceptions.

If this doesn't help, you should compile a debug version and run it with a trace file or under gdb. See section G.1 Debugging a MySQL server.

4.11.4 SunOS 4 notes

On SunOS 4, MIT-pthreads is needed to compile MySQL, which in turn means you will need GNU make.

Some SunOS 4 systems have problems with dynamic libraries and libtool. You can use the following configure line to avoid this problem:

shell> ./configure --disable-shared --with-mysqld-ldflags=-all-static

When compiling readline, you may get warnings about duplicate defines. These may be ignored.

When compiling mysqld, there will be some implicit declaration of function warnings. These may be ignored.

4.11.5 Linux notes (all Linux versions)

MySQL uses LinuxThreads on Linux. If you are using an old Linux version that doesn't have glibc2, you must install LinuxThreads before trying to compile MySQL. http://www.mysql.com/Downloads/Linux

If you can't start mysqld or if mysql_install_db doesn't work, please continue reading! This only happens on Linux system with problems in the LinuxThreads or libc/glibc libraries. There are a lot of simple workarounds to get MySQL to work! The simplest is to use the binary version of MySQL (not the RPM) for Linux x86. One nice aspect of this version is that it's probably 10% faster than any version you would compile yourself! See section 10.3 How compiling and linking affects the speed of MySQL.

One known problem with the binary distribution is that with older Linux systems that use libc (like RedHat 4.x or Slackware), you will get some non-fatal problems with hostname resolution See section 4.6.3.1 Linux notes.

isamchk hangs with libc.so.5.3.12. Upgrading to the newest libc fixes this problem.

When using LinuxThreads you will see a minimum of three processes running. These are in fact threads. There will be one thread for the LinuxThreads manager, one thread to handle connections, and one thread to handle alarms and signals.

If you see a dead mysqld daemon process with ps, this usually means that you have found a bug in MySQL or you have got a corrupted table. See section 18.1 What to do if MySQL keeps crashing.

If you are using LinuxThreads and mysqladmin shutdown doesn't work, you must upgrade to LinuxThreads 0.7.1 or newer.

If you are using RedHat, you might get errors like this:

/usr/bin/perl is needed...
/usr/sh is needed...
/usr/sh is needed...

If so, you should upgrade your version of rpm to `rpm-2.4.11-1.i386.rpm' and `rpm-devel-2.4.11-1.i386.rpm' (or later).

You can get the upgrades of libraries to RedHat 4.2 from ftp://ftp.redhat.com/updates/4.2/i386. Or http://www.sunsite.unc.edu/pub/Linux/distributions/redhat/code/rpm/ for other distributions.

If you are linking your own MySQL client and get the error:

ld.so.1: ./my: fatal: libmysqlclient.so.4: open failed: No such file or directory

when executing them, the problem can be avoided by one of the following methods:

If you are using the Fujitsu compiler (fcc / FCC) you will have some problems compiling MySQL because the Linux header files are very gcc oriented.

The following configure line should work with fcc/FCC:

CC=fcc CFLAGS="-O -K fast -K lib -K omitfp -Kpreex -D_GNU_SOURCE -DCONST=const -DNO_STRTOLL_PROTO" CXX=FCC CXXFLAGS="-O -K fast -K lib  -K omitfp -K preex --no_exceptions --no_rtti -D_GNU_SOURCE -DCONST=const -Dalloca=__builtin_alloca -DNO_STRTOLL_PROTO '-D_EXTERN_INLINE=static __inline'" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static --disable-shared --with-low-memory

4.11.5.1 Linux-x86 notes

MySQL requires libc version 5.4.12 or newer. It's known to work with libc 5.4.46. glibc version 2.0.6 and later should also work. There have been some problems with the glibc RPMs from RedHat so if you have problems, check whether or not there are any updates! The glibc 2.0.7-19 and 2.0.7-29 RPMs are known to work.

On some older Linux distributions, configure may produce an error like this:

Syntax error in sched.h. Change _P to __P in the /usr/include/sched.h file.
See the Installation chapter in the Reference Manual.

Just do what the error message says and add an extra underscore to the _P macro that has only one underscore, then try again.

You may get some warnings when compiling; those shown below can be ignored:

mysqld.cc -o objs-thread/mysqld.o
mysqld.cc: In function `void init_signals()':
mysqld.cc:315: warning: assignment of negative value `-1' to `long unsigned int'
mysqld.cc: In function `void * signal_hand(void *)':
mysqld.cc:346: warning: assignment of negative value `-1' to `long unsigned int'

In Debian GNU/Linux, if you want MySQL to start automatically when the system boots, do the following:

shell> cp support-files/mysql.server /etc/init.d/mysql.server
shell> /usr/sbin/update-rc.d mysql.server defaults 99

mysql.server can be found in the `share/mysql' directory under the MySQL installation directory, or in the `support-files' directory of the MySQL source tree.

If mysqld always core dumps when it starts up, the problem may be that you have an old `/lib/libc.a'. Try renaming it, then remove `sql/mysqld' and do a new make install and try again. This problem has been reported on some Slackware installations. RedHat 5.0 has also a similar problem with some new glibc versions. See section 4.11.5.2 RedHat 5.0 notes.

If you get the following error when linking mysqld, it means that your `libg++.a' is not installed correctly:

/usr/lib/libc.a(putc.o): In function `_IO_putc':
putc.o(.text+0x0): multiple definition of `_IO_putc'

You can avoid using `libg++.a' by running configure like this:

shell> CXX=gcc ./configure

4.11.5.2 RedHat 5.0 notes

If you have any problems with MySQL on RedHat, you should start by upgrading glibc to the newest possible version!

If you install all the official RedHat patches (including glibc-2.0.7-19 and glibc-devel-2.0.7-19), both the binary and source distributions of MySQL should work without any trouble!

The updates are needed since there is a bug in glibc 2.0.5 in how pthread_key_create variables are freed. With glibc 2.0.5, you must use a statically-linked MySQL binary distribution. If you want to compile from source, you must install the corrected version of LinuxThreads from http://www.mysql.com/Downloads/Linux or upgrade your glibc.

If you have an incorrect version of glibc or LinuxThreads, the symptom is that mysqld crashes after each connection. For example, mysqladmin version will crash mysqld when it finishes!

Another symptom of incorrect libraries is that mysqld crashes at once when it starts. On some Linux systems, this can be fixed by configuring like this:

shell> ./configure --with-mysqld-ldflags=-all-static

On Redhat 5.0, the easy way out is to install the glibc 2.0.7-19 RPM and run configure without the --with-mysqld-ldflags=-all-static option.

For the source distribution of glibc 2.0.7, a patch that is easy to apply and is tested with MySQL may be found at:

http://www.mysql.com/Download/Linux/glibc-2.0.7-total-patch.tar.gz

If you experience crashes like these when you build MySQL, you can always download the newest binary version of MySQL. This is statically-linked to avoid library conflicts and should work on all Linux systems!

MySQL comes with an internal debugger that can generate trace files with a lot of information that can be used to find and solve a wide range of different problems. See section G.1 Debugging a MySQL server.

4.11.5.3 RedHat 5.1 notes

The glibc of RedHat 5.1 (glibc 2.0.7-13) has a memory leak, so to get a stable MySQL version, you must upgrade glibc to 2.0.7-19, downgrade glibc or use a binary version of mysqld. If you don't do this, you will encounter memory problems (out of memory, etc., etc.). The most common error in this case is:

Can't create a new thread (errno 11). If you are not out of available
memory, you can consult the manual for any possible OS dependent bug

After you have upgraded to glibc 2.0.7-19, you can configure MySQL with dynamic linking (the default), but you cannot run configure with the --with-mysqld-ldflags=-all-static option until you have installed glibc 2.0.7-19 from source!

You can check which version of glibc you have with rpm -q glibc.

4.11.5.4 Linux-SPARC notes

In some implementations, readdir_r() is broken. The symptom is that SHOW DATABASES always returns an empty set. This can be fixed by removing HAVE_READDIR_R from `config.h' after configuring and before compiling.

Some problems will require patching your Linux installation. The patch can be found at http://www.mysql.com/patches/Linux-sparc-2.0.30.diff. This patch is against the Linux distribution `sparclinux-2.0.30.tar.gz' that is available at vger.rutgers.edu (a version of Linux that was never merged with the official 2.0.30). You must also install LinuxThreads 0.6 or newer.

Thanks to jacques@solucorp.qc.ca for this information.

4.11.5.5 Linux-Alpha notes

The big problem on Linux-Alpha is that there are still some problems with threads in glibc on this platform. You should start by getting the newest glibc version you can find.

Note that before you run any programs that use threads (like mysqld, thr_alarm or thr_lock), you should raise the shared memory limit (with ulimit). The MySQL benchmarks are known to fail if you forget to do this!

Configure MySQL with the following command:

shell> CC=gcc CCFLAGS="-Dalpha_linux_port" \
       CXX=gcc CXXFLAGS="-O3 -Dalpha_linux_port -felide-constructors -fno-exceptions -fno-rtti" \
       ./configure --prefix=/usr/local/mysql

Try to compile mysys/thr_lock and mysys/thr_alarm. Test that these programs work! (Invoke each one with no arguments. Each should end with test_succeeded if everything was okay.)

After installing MySQL, uncomment the ulimit command in safe_mysqld and add options to increase shared memory.

Note that Linux-Alpha is still an alpha-quality platform for MySQL. With the newest glibc, you have a very good chance of it working.

If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:

shell> CFLAGS=-DDONT_USE_THR_ALARM \
       CXXFLAGS=-DDONT_USE_THR_ALARM \
       ./configure ...

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are "sleeping" on a connection with mysqladmin kill or mysqladmin shutdown. Instead, the client will die when it issues its next command.

4.11.5.6 MkLinux notes

MySQL should work on MkLinux with the newest glibc package (tested with glibc 2.0.7).

4.11.5.7 Qube2 Linux notes

To get MySQL to work on Qube2, (Linux Mips), you need the newest glibc libraries (glibc-2.0.7-29C2 is known to work). You must also use the egcs C++ compiler (egcs-1.0.2-9 or newer).

4.11.6 Alpha-DEC-Unix notes

When compiling threaded programs under Digital UNIX, the documentation recommends using the -pthread option for cc and cxx and the libraries -lmach -lexc (in addition to -lpthread). You should run configure something like this:

shell> CC="cc -pthread" CXX="cxx -pthread -O" \
       ./configure --with-named-thread-libs="-lpthread -lmach -lexc -lc"

When compiling mysqld, you may see a couple of warnings like this:

mysqld.cc: In function void handle_connections()':
mysqld.cc:626: passing long unsigned int *' as argument 3 of
accept(int,sockadddr *, int *)'

You can safely ignore these warnings. They occur because configure can detect only errors, not warnings.

If you start the server directly from the command line, you may have problems with it dying when you log out. (When you log out, your outstanding processes receive a SIGHUP signal.) If so, try starting the server like this:

shell> nohup mysqld [options] &

nohup causes the command following it to ignore any SIGHUP signal sent from the terminal. Alternatively, start the server by running safe_mysqld, which invokes mysqld using nohup for you.

4.11.7 Alpha-DEC-OSF1 notes

If you have problems compiling and have DEC CC and gcc installed, try running configure like this:

shell> CC=cc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 \
       ./configure --prefix=/usr/local/mysql

If you get problems with the `c_asm.h' file, you can create and use a 'dummy' `c_asm.h' file with:

shell> touch include/c_asm.h
shell> CC=gcc CFLAGS=-I./include \
       CXX=gcc CXXFLAGS=-O3 \
       ./configure --prefix=/usr/local/mysql

On OSF1 V4.0D and compiler "DEC C V5.6-071 on Digital UNIX V4.0 (Rev. 878)" the compiler had some strange behavior (undefined asm symbols). /bin/ld also appears to be broken (problems with _exit undefined errors occuring while linking mysqld). On this system, we have managed to compile MySQL with the following configure line, after replacing /bin/ld with the version from OSF 4.0C:

shell> CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql

In some versions of OSF1, the alloca() function is broken. Fix this by removing the line in `config.h' that defines 'HAVE_ALLOCA'.

The alloca() function also may have an incorrect prototype in /usr/include/alloca.h. This warning resulting from this can be ignored.

configure will use the following thread libraries automatically: --with-named-thread-libs="-lpthread -lmach -lexc -lc".

When using gcc, you can also try running configure like this:

shell> CFLAGS=-D_PTHREAD_USE_D4 CXX=gcc CXXFLAGS=-O3 ./configure ....

If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:

shell> CFLAGS=-DDONT_USE_THR_ALARM \
       CXXFLAGS=-DDONT_USE_THR_ALARM \
       ./configure ...

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are "sleeping" on a connection with mysqladmin kill or mysqladmin shutdown. Instead, the client will die when it issues its next command.

4.11.8 SGI-Irix notes

You may have to undefine some things in `config.h' after running configure and before compiling.

In some Irix implementations, the alloca() function is broken. If the mysqld server dies on some SELECT statements, remove the lines from `config.h' that define HAVE_ALLOC and HAVE_ALLOCA_H. If mysqladmin create doesn't work, remove the line from `config.h' that defines HAVE_READDIR_R. You may have to remove the HAVE_TERM_H line as well.

SGI recommends that you install all of the patches on this page as a set: http://support.sgi.com/surfzone/patches/patchset/6.2_indigo.rps.html

At the very minimum, you should install the latest kernel rollup, the latest rld rollup, and the latest libc rollup.

You definately need all the POSIX patches on this page, for pthreads support:

http://support.sgi.com/surfzone/patches/patchset/6.2_posix.rps.html

If you get the something like the following error when compiling `mysql.cc':

"/usr/include/curses.h", line 82: error(1084): invalid combination of type

Then type the following in the top-level directory of your MySQL source tree:

shell> extra/replace bool curses_bool < /usr/include/curses.h > include/curses.h
shell> make

There have also been reports of scheduling problems. If only one thread is running, things go slow. Avoid this by starting another client. This may lead to a 2-to-10-fold increase in execution speed thereafter for the other thread. This is a poorly-understood problem with Irix threads; you may have to improvise to find solutions until this can be fixed.

If you are compiling with gcc, you can use the following configure command:

shell> CC=gcc CXX=gcc CXXFLAGS=-O3 \
       ./configure --prefix=/usr/local/mysql --with-thread-safe-client --with-named-thread-libs=-lpthread

4.11.9 FreeBSD notes

If you notice that configure will use MIT-pthreads, you should read the MIT-pthreads notes. See section 4.9 MIT-pthreads notes.

If you get an error from make install that it can't find `/usr/include/pthreads', configure didn't detect that you need MIT-pthreads. This is fixed by executing these commands:

shell> rm config.cache
shell> ./configure --with-mit-threads

The behavior of FreeBSD make is slightly different from that of GNU make. If you have make-related problems, you should install GNU make.

If mysql or mysqladmin takes a long time to respond, a user said the following:

Are you running the ppp user process? On one FreeBSD box (2.2.5) MySQL clients takes a couple of seconds to connect to mysqld if the ppp process is running.

FreeBSD is also known to have a very low default file handle limit. See section 18.11 File not found.

If you have a problem with SELECT NOW() returning values in GMT and not your local time, you have to set the TZ environment variable to your current timezone. This should be done for the environment in which the server runs, for example, in safe_mysqld or mysql.server.

Make sure that the localhost entry in the `/etc/hosts' file is correct (otherwise you will have problems connecting to the database). The `/etc/hosts' file should start with a line:

127.0.0.1       localhost localhost.your.domain

If you are using FreeBSD 2.2.6, don't forget to apply the ttcp and mmap-22 patches to the OS (for security reasons). Please see http://www.freebsd.org for these CERT patches.

If you are using FreeBSD 2.2.7 and you have problems killing the mysqld daemon, you should get new sources using cvsup and recompile libc_r.

4.11.9.1 FreeBSD-3.0 notes

You have to run configure with the --with-named-thread-libs=-lc_r option.

The pthreads library for FreeBSD doesn't contain the sigwait() function and there are some bugs in it. To fix this, get the `FreeBSD-3.0-libc_r-1.0.diff' file from the FreeBSD area of the MySQL FTP site and apply it in the `/usr/src/lib/libc_r/uthread' directory. Then follow the instructions that can be found with man pthread about how to recompile the libc_r library.

You can test if you have a "modern" libpthread.a with this command:

shell> nm /usr/lib/libc_r.a | grep sigwait

If the above doesn't find sigwait, you must use the patch above and recompile libc_r.

4.11.10 NetBSD notes

To compile on NetBSD you need GNU make. Otherwise the compile will crash when make tries to run lint on C++ files.

4.11.11 BSD/OS notes

4.11.11.1 BSD/OS 2.x notes

If you get the following error when compiling MySQL, your ulimit value for virtual memory is too low:

item_func.h: In method `Item_func_ge::Item_func_ge(const Item_func_ge &)':
item_func.h:28: virtual memory exhausted
make[2]: *** [item_func.o] Error 1

Try using ulimit -v 80000 and run make again. If this doesn't work and you are using bash, try switching to csh or sh; some BSDI users have reported problems with bash and ulimit.

If you are using gcc, you may also use have to use the --with-low-memory flag for configure to be able to compile `sql_yacc.cc'.

If you have a problem with SELECT NOW() returning values in GMT and not your local time, you have to set the TZ environment variable to your current timezone. This should be done for the environment in which the server runs, for example in safe_mysqld or mysql.server.

4.11.11.2 BSD/OS 3.x notes

Upgrade to BSD/OS 3.1. If that is not possible, install BSDIpatch M300-038.

Use the following command when configuring MySQL:

shell> env CXX=shlicc++ CC=shlicc2 \
       ./configure \
           --prefix=/usr/local/mysql \
           --localstatedir=/var/mysql \
           --without-perl \
           --with-unix-socket-path=/var/mysql/mysql.sock

The following is also known to work:

shell> env CC=gcc CXX=gcc CXXFLAGS=-O3 \
       ./configure \
           --prefix=/usr/local/mysql \
           --with-unix-socket-path=/var/mysql/mysql.sock

You can change the directory locations if you wish, or just use the defaults by not specifying any locations.

If you have problems with performance under heavy load, try using the --skip-thread-priority option to safe_mysqld! This will run all threads with the same priority; on BSDI 3.1, this gives better performance (at least until BSDI fixes their thread scheduler).

If you get the error virtual memory exhausted while compiling, you should try using ulimit -v 80000 and run make again. If this doesn't work and you are using bash, try switching to csh or sh; some BSDI users have reported problems with bash and ulimit.

4.11.11.3 BSD/OS 4.x notes

BSDI 4.x has some thread related bugs. If you want to use MySQL on this, you should install all thread related patches. At least M400-023 should be installed.

4.11.12 SCO notes

The current port is tested only on a "sco3.2v5.0.4" and "sco3.2v5.0.5" system. There has also been a lot of progress on a port to "sco 3.2v4.2".

  1. For OpenServer 5.0.X you need to use GDS in Skunkware 95 (95q4c). This is necessary because GNU gcc 2.7.2 in Skunkware 97 does not have GNU as. You can also use egcs 1.1.2 or newer http://www.egcs.com/. If you are using egcs 1.1.2 you have to execute the following command:
    shell> cp -p /usr/include/pthread/stdtypes.h /usr/local/lib/gcc-lib/i386-pc-sco3.2v5.0.5/egcs-2.91.66/include/pthread/
    
  2. You need the port of GCC 2.5.? for this product and the Development system. They are required on this version of SCO UNIX. You cannot just use the GCC Dev system.
  3. You should get the FSU Pthreads package and install it first. This can be found at http://www.cs.wustl.edu/~schmidt/ACE_wrappers/FSU-threads.tar.gz. You can also get a precompiled package from ftp://www.mysql.com/pub/mysql/Downloads/SCO/FSU-threads-3.5c.tar.gz.
  4. FSU Pthreads can be compiled with SCO UNIX 4.2 with tcpip. Or OpenServer 3.0 or Open Desktop 3.0 (OS 3.0 ODT 3.0), with the SCO Development System installed using a good port of GCC 2.5.X ODT or OS 3.0 you will need a good port of GCC 2.5.? There are a lot of problems without a good port. The port for this product requires the SCO UNIX Development system. Without it, you are missing the libraries and the linker that is needed.
  5. To build FSU Pthreads on your system, do the following:
    1. Run ./configure in the `threads/src' directory and select the SCO OpenServer option. This command copies `Makefile.SCO5' to `Makefile'.
    2. Run make.
    3. To install in the default `/usr/include' directory, login as root, then cd to the `thread/src' directory, and run make install.
  6. Remember to use GNU make when making MySQL.
  7. On OSR 5.0.5, you should use the following configure line:
    shell> CC="gcc -DSCO" CXX="gcc -DSCO" ./configure
    
    The -DSCO is needed to help configure detect some thread functions properly. If you forget -DSCO, you will get the following error message while compiling:
    my_pthread.c: In function `my_pthread_mutex_init':
    my_pthread.c:374: `pthread_mutexattr_default' undeclared (first use this function)
    
  8. If you don't start safe_mysqld as root, you probably will get only the default 110 open files per process. mysqld will write a note about this in the log file.
  9. With SCO 3.2V5.0.5, you should use a FSU Pthreads version 3.5c or newer. The following configure command should work:
    shell> CC="gcc -belf" ./configure --prefix=/usr/local/mysql --disable-shared
    
  10. With SCO 3.2V4.2, you should use a FSU Pthreads version 3.5c or newer. The following configure command should work:
    shell> CFLAGS="-D_XOPEN_XPG4" CXX=gcc CXXFLAGS="-D_XOPEN_XPG4" \
           ./configure \
               --with-debug --prefix=/usr/local/mysql \
               --with-named-thread-libs="-lgthreads -lsocket -lgen -lgthreads" \
               --with-named-curses-libs="-lcurses"
    
    You may get some problems with some include files. In this case, you can find new SCO-specific include files at ftp://www.mysql.com/pub/mysql/Downloads/SCO/SCO-3.2v4.2-includes.tar.gz. You should unpack this file in the `include' directory of your MySQL source tree.

SCO development notes:

If you want to install DBI on SCO, you have to edit the `Makefiles' in DBI-xxx and each subdirectory:

OLD:                                  NEW:
CC = cc                               CC = gcc -belf
CCCDLFLAGS = -KPIC -W1,-Bexport       CCCDLFLAGS = -fpic
CCDLFLAGS = -wl,-Bexport              CCDLFLAGS =

LD = ld                               LD = gcc -belf -G -fpic
LDDLFLAGS = -G -L/usr/local/lib       LDDLFLAGS = -L/usr/local/lib
LDFLAGS = -belf -L/usr/local/lib      LDFLAGS = -L/usr/local/lib

LD = ld                               LD = gcc -belf -G -fpic
OPTIMISE = -Od         		      OPTIMISE = -O1

OLD:
CCCFLAGS = -belf -dy -w0 -U M_XENIX -DPERL_SCO5 -I/usr/local/include

NEW:
CCFLAGS = -U M_XENIX -DPERL_SCO5 -I/usr/local/include

This is because the Perl dynaloader will not load the DBI modules if they were compiled with icc or cc.

Perl works best when compiled with cc.

4.11.13 SCO Unixware 7.0 notes

You must use a version of MySQL at least as recent as 3.22.13, since that version fixes some portability problems under Unixware.

We have been able to compile MySQL with the following configure command on UnixWare 7.0.1:

shell> CC=cc CXX=CC ./configure --prefix=/usr/local/mysql

4.11.14 IBM-AIX notes

Automatic detection of xlC is missing from Autoconf, so a configure command something like this is needed when using the IBM compiler:

shell> CC="xlc_r -ma -O3 -qstrict -DHAVE_INT_8_16_32" \
       CXX="xlC_r -ma -O3 -qstrict -DHAVE_INT_8_16_32" \
       ./configure

If you are using egcs to compile MySQL, you MUST use the -fno-exceptions flag, as the exception handling in egcs is not thread-safe! (This is tested with egcs 1.1.) We recommend the following configure line with egcs and gcc on AIX:

shell> CXX=gcc \
       CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti" \
       ./configure --prefix=/home/monty --with-debug --with-low-memory

If you have problems with signals (MySQL dies unexpectedly under high load) you may have found an OS bug with threads and signals. In this case you can tell MySQL not to use signals by configuring with:

shell> CFLAGS=-DDONT_USE_THR_ALARM CXX=gcc \
       CXXFLAGS="-felide-constructors -fno-exceptions -fno-rtti -DDONT_USE_THR_ALARM" \
       ./configure --prefix=/home/monty --with-debug --with-low-memory

This doesn't affect the performance of MySQL, but has the side effect that you can't kill clients that are "sleeping" on a connection with mysqladmin kill or mysqladmin shutdown. Instead, the client will die when it issues its next command.

4.11.15 HP-UX notes

There are a couple of "small" problems when compiling MySQL on HP-UX. We recommend that you use gcc instead of the HP-UX native compiler, because gcc produces better code!

We recommend one to use gcc 2.95 on HP-UX. Don't use high optimization flags (like -O6) as this may not be safe on HP-UX.

Note that MIT-pthreads can't be compiled with the HP-UX compiler, because it can't compile .S (assembler) files.

The following configure line should work:

CFLAGS="-DHPUX -I/opt/dce/include" CXXFLAGS="-DHPUX -I/opt/dce/include -felide-constructors -fno-exceptions -fno-rtti" CXX=gcc ./configure --with-pthread --with-named-thread-libs='-ldce' --prefix=/usr/local/mysql --disable-shared

If you are compiling gcc 2.95 yourself, you should NOT link it with the DCE libraries (libdce.a or libcma.a) if you want to compile MySQL with MIT-pthreads. If you mix the DCE and MIT-pthreads packages you will get a mysqld to which you cannot connect. Remove the DCE libraries while you compile gcc 2.95!

4.12 Win32 notes

This section describes installation and use of MySQL on Win32. This is also described in the `README' file that comes with the MySQL Win32 distribution.

4.12.1 Installing MySQL on Win32

If you don't have a registered version of MySQL, you should first download the shareware version from:

MySQL 3.21.29

If you plan to connect to MySQL from some other program, you will probably also need the MyODBC driver. You can find this at the MySQL download page.

To install either distribution, unzip it in some empty directory and run the Setup.exe program.

Installation takes place in `C:\mysql'.

4.12.2 Starting MySQL on Win95 / Win98

MySQL uses TCP/IP to connect a client to a server. (This will allow any machine on your network to connect to your MySQL server). Because of this, you must install TCP/IP on your machine before starting MySQL. You can find TCP/IP on your Windows CD-ROM.

Note that if you are using an old Win95 release (for example OSR2), it's likely that you have an old Winsock package! MySQL requires Winsock 2! You can get the newest Winsock from Microsoft. Win98 has as default the new Winsock 2 library, so the above doesn't apply for Win98.

There are 2 different MySQL servers you can use:

mysqld Compiled with full debugging and automatic memory allocation checking
mysqld-opt Optimized for a Pentium processor.

Both of the above should work on any Intel processor >= i386.

To start the mysqld server, you should start a MS-DOS window and type:

C:\mysql\bin\mysqld

This will start mysqld in the background without a window.

You can kill the MySQL server by executing:

C:\mysql\bin\mysqladmin -u root shutdown

Note that Win95/Win98 don't support creation of named pipes. On Win95/Win98, you can only use named pipes to connect to a remote MySQL running on an NT server.

4.12.3 Starting MySQL on NT

The Win95/Win98 section also applies to MySQL on NT, with the following differences:

To get MySQL to work with TCP/IP, you must install service pack 3 (or newer)!

For NT, the server name is mysqld-nt. Normally you should install MySQL as a service on NT:

C:\mysql\bin\mysqld-nt --install

(You could use the mysqld or mysqld-opt servers on NT, but those cannot be started as a service or use named pipes.)

You can start and stop the MySQL service with:

NET START mysql
NET STOP mysql

Note that in this case you can't use any other options for mysqld-nt!

You can also run mysqld-nt as a standalone program on NT if you need to start mysqld-nt with any options! If you start mysqld-nt without options on NT, mysqld-nt tries to starts itself as a service with the default service options. If you have stopped mysqld-nt, you have to start it with NET START mysql.

The service is installed with the name MySql. Once installed, it must be started using Services Control Manager (SCM) Utility (found in Control Panel) or by using the NET START MySQL command. If any options are desired, they must be specified as "Startup parameters" in the SCM utility before you start the MySQL service. Once running, mysqld-nt can be stopped using mysqladmin or from the SCM utility or by using the command NET STOP MySQL. If you use SCM to stop mysqld-nt, there is a strange message from SCM about mysqld shutdown normally. When run as a service, mysqld-nt has no access to a console and so no messages can be seen.

On NT you can get the following service error messages:

Permission Denied Means that it cannot find mysqld-nt.exe
Cannot Register Means that the path is incorrect

If you have problems installing mysqld-nt as a service, try starting it with the full path:

C:\mysql\bin\mysqld --install

If this doesn't work, you can get mysqld-nt to start properly by fixing the path in the registry!

If you don't want to start mysqld-nt as a service, you can start it as follows:

C:\mysql\bin\mysqld-nt --standalone

or

C:\mysql\bin\mysqld-nt --standalone --debug

The last version gives you a debug trace in `C:\mysqld.trace'.

4.12.4 Running MySQL on Win32

MySQL supports TCP/IP on all Win32 platforms and named pipes on NT. The default is to use named pipes for local connections on NT and TCP/IP for all other cases if the client has TCP/IP installed. The host name specifies which protocol is used:

Host name
protocol
NULL (none) On NT, try named pipes first; if that doesn't work, use TCP/IP. On Win95/Win98, TCP/IP is used.
. Named pipes
localhost TCP/IP to current host
hostname TCP/IP

You can force a MySQL client to use named pipes by specifying the --pipe option. Use the --socket option to specify the name of the pipe.

You can test whether or not MySQL is working by executing the following commands:

C:\mysql\bin\mysqlshow
C:\mysql\bin\mysqlshow -u root mysql
C:\mysql\bin\mysqladmin version status proc
C:\mysql\bin\mysql test

By default, MySQL-Win32 is configured to be installed in `C:\mysql'. If you want to install MySQL elsewhere, install it in `C:\mysql', then move the installation to where you want it. If you do move MySQL, you must tell mysqld where everything is by supplying options to mysqld. Use C:\mysql\bin\mysqld --help to display all options! If, for example, you have moved the MySQL distribution to `D:\programs\mysql', you must start mysqld with: D:\programs\mysql\bin\mysqld --basedir D:\programs\mysql

With the registered version of MySQL, you can also create a `C:\my.cnf' file that holds any default options for the MySQL server. Copy the file `\mysql\my-example.cnf' to `C:\my.cnf' and edit this to suit your setup. Note that you should specify all paths with / instead of \. If you use \, you need to specify this twice, as \ is the escape character in MySQL. See section 4.15.4 Option files.

If mysqld is slow to answer to connections on Win95/Win98, there is probably a problem with your DNS. In this case, start mysqld with --skip-name-resolve and use only localhost and IP numbers in the MySQL grant tables. You can also avoid DNS when connecting to a mysqld-nt MySQL server running on NT by using the --pipe argument to specify use of named pipes. This works for most MySQL clients.

There are two versions of the MySQL command line tool:
mysql Compiled on native Win32, which offers very limited text editing capabilities.
mysqlc Compiled with the Cygnus GNU compiler and libraries, which offers readline editing.

If you want to use mysqlc.exe, you must copy `C:\mysql\lib\cygwinb19.dll' to `\windows\system' (or similar place).

The default privileges on Win32 give all local users full privileges to all databases. To make MySQL more secure, you should set a password for all users and remove the row in the mysql.user table that has Host='localhost' and User=".

You should also add a password for the root user:

C:\mysql\bin\mysql mysql
mysql> DELETE FROM user WHERE Host='localhost' AND User=";
mysql> QUIT
C:\mysql\bin\mysqladmin reload
C:\mysql\bin\mysqladmin -u root password your_password

After you've set the password, if you want to take down the mysqld server, you can do so using this command:

mysqladmin --user=root --password=your_password shutdown

4.12.5 Connecting to a remote MySQL from Win32 with SSH

Here is a note about how to connect to get a secure connection to remote MySQL server with SSH (by David Carlson).

That's it. It works very well with a direct Internet connection. I'm having problems with SSH conflicting with my Win95 network and Wingate - but that'll be the topic of a posting on another software company's usegroup!

4.12.6 MySQL-Win32 compared to Unix MySQL

MySQL-Win32 has by now proven itself to be very stable. This version of MySQL has the same features as the corresponding Unix version with the following exceptions:

Win95 and threads
Win95 leaks about 200 bytes of main memory for each thread creation. Because of this, you shouldn't run mysqld for an extended time on Win95 if you do many connections, since each connection in MySQL creates a new thread! WinNT and Win98 don't suffer from this bug.
Blocking read
MySQL uses a blocking read for each connection. This means that: We plan to fix this in the near future.
UDF functions
For the moment, MySQL-Win32 does not support user definable functions.
DROP DATABASE
You can't drop a database that is in use by some thread.
Killing MySQL from the task manager
You can't kill MySQL from the task manager or with the shutdown utility in Windows95. You must take it down with mysqladmin shutdown.
Case-insensitive names
Filenames are case insensitive on Win32, so database and table names are also case insensitive in MySQL for Win32. The only restriction is that database and table names must be given in the same case throughout a given statement. The following query would not work because it refers to a table both as my_table and as MY_TABLE:
SELECT * FROM my_table WHERE MY_TABLE.col=1;
The `\' directory character
Pathname components in Win95 are separated by `\' characters, which is also the escape character in MySQL. If you are using LOAD DATA INFILE or SELECT ... INTO OUTFILE, you must double the `\' character or use Unix style filenames `/' characters:
LOAD DATA INFILE "C:\\tmp\\skr.txt" INTO TABLE skr;
SELECT * FROM skr INTO OUTFILE 'C:/tmp/skr.txt';
Can't open named pipe error
If you use the shareware version of MySQL-Win32 on NT with the newest mysql-clients you will get the following error:
error 2017: can't open named pipe to host: . pipe...
This is because the release version of MySQL uses named pipes on NT by default. You can avoid this error by using the --host=localhost option to the new MySQL clients or create a file `C:\my.cnf' that contains the following information:
[client]
host = localhost
Access denied for user error
If you get the error Access denied for user: 'some-user@unknown' to database 'mysql' when accessing a MySQL server on the same machine, this means that MySQL can't resolve your host name properly. To fix this, you should create a file `\windows\hosts' with the following information:
127.0.0.1       localhost

Here are some open issues for anyone who might want to help us with the Win32 release:

Other Win32-specific issues are described in the `README' file that comes with the MySQL-Win32 distribution.

4.13 OS/2 notes

MySQL uses quite a few open files. Because of this, you should add something like the following to your `CONFIG.SYS' file:

SET EMXOPT=-c -n -h1024

If you don't do this, you will probably run into the following error:

File 'xxxx' not found (Errcode: 24)

When using MySQL with OS/2 Warp 3, FixPack 29 or above is required. With OS/2 Warp 4, FixPack 4 or above is required. This is a requirement of the Pthreads library. MySQL must be installed in a partition that supports long file names such as HPFS, FAT32, etc.

The `INSTALL.CMD' script must be run from OS/2's own `CMD.EXE' and may not work with replacement shells such as `4OS2.EXE'.

The `scripts/mysql-install-db' script has been renamed: it is now called `install.cmd' and is a REXX script which will set up the default MySQL security settings and create the WorkPlace Shell icons for MySQL.

Dynamic module support is compiled in but not fully tested. Dynamic modules should be compiled using the Pthreads runtime library.

gcc -Zdll -Zmt -Zcrtdll=pthrdrtl -I../include -I../regex -I.. \
    -o example udf_example.cc -L../lib -lmysqlclient udf_example.def
mv example.dll example.udf

Note: Due to limitations in OS/2, UDF module name stems must not exceed 8 characters. Modules are stored in the `/mysql2/udf' directory; the safe-mysqld.cmd script will put this directory in the BEGINLIBPATH environment variable. When using UDF modules, specified extensions are ignored -- it is assumed to be `.udf'. For example, in Unix, the shared module might be named `example.so' and you would load a function from it like this:

CREATE FUNCTION metaphon RETURNS STRING SONAME "example.so";

Is OS/2, the module would be named `example.udf', but you would not specify the module extension:

CREATE FUNCTION metaphon RETURNS STRING SONAME "example";

4.14 TcX binaries

As a service, TcX provides a set of binary distributions of MySQL that are compiled at TcX or at sites where customers kindly have given us access to their machines.

These distributions are generated with scripts/make_binary_distribution and are configured with the following compilers and options:

SunOS 4.1.4 2 sun4c with gcc 2.7.2.1
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --disable-shared
SunOS 5.5.1 sun4u with egcs 1.0.3a
CC=gcc CFLAGS="-O6 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O6 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory
SunOS 5.6 sun4u with egcs 2.90.27
CC=gcc CFLAGS="-O6 -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O6 -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --with-low-memory
SunOS 5.6 i86pc with gcc 2.8.1
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory
Linux 2.0.33 i386 with pgcc 2.90.29 (egcs 1.0.3a)
CFLAGS="-O6 -mpentium -mstack-align-double -fomit-frame-pointer" CXX=gcc CXXFLAGS="-O6 -mpentium -mstack-align-double -fomit-frame-pointer -felide-constructors -fno-exceptions -fno-rtti" ./configure --prefix=/usr/local/mysql --enable-assembler --with-mysqld-ldflags=-all-static
SCO 3.2v5.0.4 i386 with gcc 2.7-95q4
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
AIX 2 4 with gcc 2.7.2.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
OSF1 V4.0 564 alpha with gcc 2.8.1
CC=gcc CFLAGS=-O CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql --with-low-memory
Irix 6.3 IP32 with gcc 2.8.0
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql
BSDI BSD/OS 3.1 i386 with gcc 2.7.2.1
CC=gcc CXX=gcc CXXFLAGS=-O ./configure --prefix=/usr/local/mysql
BSDI BSD/OS 2.1 i386 with gcc 2.7.2
CC=gcc CXX=gcc CXXFLAGS=-O3 ./configure --prefix=/usr/local/mysql

Anyone who has more optimal options for any of the configurations listed above can always mail them to the developer's mailing list at

RPM distributions prior to MySQL 3.22 are user-contributed. Beginning with 3.22, some RPMs are TcX-generated.

4.15 Post-installation setup and testing

Once you've installed MySQL (from either a binary or source distribution), you need to initialize the grant tables, start the server and make sure that the server works okay. You may also wish to arrange for the server to be started and stopped automatically when your system starts up and shuts down.

Normally you install the grant tables and start the server like this:

shell> cd mysql_installation_directory
shell> ./bin/mysql_install_db
shell> ./bin/safe_mysqld &

Testing is most easily done from the top-level directory of the MySQL distribution. For a binary distribution, this is your installation directory (typically something like `/usr/local/mysql'). For a source distribution, this is the main directory of your MySQL source tree.

In the commands shown below in this section and in the following subsections, BINDIR is the path to the location in which programs like mysqladmin and safe_mysqld are installed. For a binary distribution, this is the `bin' directory within the distribution. For a source distribution, BINDIR is probably `/usr/local/bin', unless you specified an installation directory other than `/usr/local' when you ran configure. EXECDIR is the location in which the mysqld server is installed. For a binary distribution, this is the same as BINDIR. For a source distribution, EXECDIR is probably `/usr/local/libexec'.

Testing is described in detail below:

  1. If necessary, start the mysqld server and set up the initial MySQL grant tables containing the privileges that determine how users are allowed to connect to the server. This is normally done with the mysql_install_db script:
    shell> scripts/mysql_install_db
    
    Typically, mysql_install_db needs to be run only the first time you install MySQL. Therefore, if you are upgrading an existing installation, you can skip this step. (However, mysql_install_db is quite safe to use and will not update any tables that already exist, so if you are unsure what to do, you can always run mysql_install_db.) mysql_install_db creates six tables (user, db, host, tables_priv, columns_priv and func) in the mysql database. A description of the initial privileges is given in section 6.10 Setting up the initial MySQL privileges. Briefly, these privileges allow the MySQL root user to do anything, and allow anybody to create or use databases with a name of 'test' or starting with 'test_'. If you don't set up the grant tables, the following error will appear in the log file when you start the server:
    mysqld: Can't find file: 'host.frm'
    
    The above may also happens with a binary MySQL distribution if you don't start MySQL by executing exactly ./bin/safe_mysqld! You might need to run mysql_install_db as root. However, if you prefer, you can run the MySQL server as an unprivileged (non-root) user, provided that user can read and write files in the database directory. Instructions for running MySQL as an unprivileged user are given in section 18.8 How to run MySQL as a normal user. If you have problems with mysql_install_db, see section 4.15.1 Problems running mysql_install_db. There are some alternatives to running the mysql_install_db script as it is provided in the MySQL distribution: For more information about these alternatives, see section 6.10 Setting up the initial MySQL privileges.
  2. Start the MySQL server like this:
    shell> cd mysql_installation_directory
    shell> bin/safe_mysqld &
    
    If you have problems starting the server, see section 4.15.2 Problems starting the MySQL server.
  3. Use mysqladmin to verify that the server is running. The following commands provide a simple test to check that the server is up and responding to connections:
    shell> BINDIR/mysqladmin version
    shell> BINDIR/mysqladmin variables
    
    The output from mysqladmin version varies slightly depending on your platform and version of MySQL, but should be similar to that shown below:
    shell> BINDIR/mysqladmin version
    mysqladmin  Ver 6.3 Distrib 3.22.9-beta, for pc-linux-gnu on i686
    TCX Datakonsult AB, by Monty
    
    Server version          3.22.9-beta
    Protocol version        10
    Connection              Localhost via UNIX socket
    TCP port                3306
    UNIX socket             /tmp/mysql.sock
    Uptime:                 16 sec
    
    Running threads: 1  Questions: 20  Reloads: 2  Open tables: 3
    
    To get a feeling for what else you can do with BINDIR/mysqladmin, invoke it with the --help option.
  4. Verify that you can shut down the server:
    shell> BINDIR/mysqladmin -u root shutdown
    
  5. Verify that you can restart the server. Do this using safe_mysqld or by invoking mysqld directly. For example:
    shell> BINDIR/safe_mysqld --log &
    
    If safe_mysqld fails, try running it from the MySQL installation directory (if you are not already there). If that doesn't work, see section 4.15.2 Problems starting the MySQL server.
  6. Run some simple tests to verify that the server is working. The output should be similar to what is shown below:
    shell> BINDIR/mysqlshow
    +-----------+
    | Databases |
    +-----------+
    | mysql     |
    +-----------+
    
    shell> BINDIR/mysqlshow mysql
    Database: mysql
    +--------------+
    |    Tables    |
    +--------------+
    | columns_priv |
    | db           |
    | func         |
    | host         |
    | tables_priv  |
    | user         |
    +--------------+
    
    shell> BINDIR/mysql -e "select host,db,user from db" mysql
    +------+--------+------+
    | host | db     | user |
    +------+--------+------+
    | %    | test   |      |
    | %    | test_% |      |
    +------+--------+------+
    
    There is also a benchmark suite in the `sql-bench' directory (under the MySQL installation directory) that you can use to compare how MySQL performs on different platforms. The `sql-bench/Results' directory contains the results from many runs against different databases and platforms. To run all tests, execute these commands:
    shell> cd sql-bench
    shell> run-all-tests
    
    If you don't have the `sql-bench' directory, you are probably using an RPM for a binary distribution. (Source distribution RPMs include the benchmark directory.) In this case, you must first install the benchmark suite before you can use it. Beginning with MySQL 3.22, there are benchmark RPM files named `mysql-bench-VERSION-i386.rpm' that contain benchmark code and data. If you have a source distribution, you can also run the tests in the `tests' subdirectory. For example, to run `auto_increment.tst', do this:
    shell> BINDIR/mysql -vvf test < ./tests/auto_increment.tst
    
    The expected results are shown in the `./tests/auto_increment.res' file.

4.15.1 Problems running mysql_install_db

This section lists problems you might encounter when you run mysql_install_db:

mysql_install_db doesn't install the grant tables
You may find that mysql_install_db fails to install the grant tables and terminates after displaying the following messages:
starting mysqld daemon with databases from XXXXXX
mysql daemon ended
In this case, you should examine the log file very carefully! The log should be located in the directory `XXXXXX' named by the error message, and should indicate why mysqld didn't start. If you don't understand what happened, include the log when you post a bug report using mysqlbug! See section 2.3 How to report bugs or problems.
There is already a mysqld daemon running
In this case, you have probably don't have to run mysql_install_db at all. You have to run mysql_install_db only once, when you install MySQL the first time.
Installing a second mysqld daemon doesn't work when one daemon is running
This can happen when you already have an existing MySQL installation, but want to put a new installation in a different place (e.g., for testing, or perhaps you simply want to run two installations at the same time). Generally the problem that occurs when you try to run the second server is that it tries to use the same socket and port as the old one. In this case you will get the error message: Can't start server: Bind on TCP/IP port: Address already in use or Can't start server : Bind on unix socket... You can start the new server with a different socket and port as follows:
shell> MYSQL_UNIX_PORT=/tmp/mysqld-new.sock
shell> MYSQL_TCP_PORT=3307
shell> export MYSQL_UNIX_PORT MYSQL_TCP_PORT
shell> scripts/mysql_install_db
shell> bin/safe_mysqld &
After this, you should edit your server boot script to start both daemons with different sockets and ports. For example, it could invoke safe_mysqld twice, but with different --socket, --port and --basedir options for each invocation.
You don't have write access to `/tmp'
If you don't have write access to create a socket file at the default place (in `/tmp') or permission to create temporary files in `/tmp,' you will get an error when running mysql_install_db or when starting or using mysqld. You can specify a different socket and temporary directory as follows:
shell> TMPDIR=/some_tmp_dir/
shell> MYSQL_UNIX_PORT=/some_tmp_dir/mysqld.sock
shell> export TMPDIR MYSQL_UNIX_PORT
`some_tmp_dir' should be the path to some directory for which you have write permission. After this you should be able to run mysql_install_db and start the server with these commands:
shell> scripts/mysql_install_db
shell> BINDIR/safe_mysqld &
mysqld crashes immediately
If you are running RedHat 5.0 with a version of glibc older than 2.0.7-5, you should make sure you have installed all glibc patches! There is a lot of information about this in the MySQL mail archives. Links to the mail archives are available at the online MySQL documentation page. Also, see section 4.11.5 Linux notes (all Linux versions). You can also start mysqld manually using the --skip-grant option and add the privilege information yourself using mysql:
shell> BINDIR/safe_mysqld --skip-grant &
shell> BINDIR/mysql -u root mysql
From mysql, manually execute the SQL commands in mysql_install_db. Make sure you run mysqladmin reload afterward to tell the server to reload the grant tables.

4.15.2 Problems starting the MySQL server

Generally, you start the mysqld server in one of three ways:

Whichever method you use to start the server, if it fails to start up correctly, check the log file to see if you can find out why. Log files are located in the data directory (typically `/usr/local/mysql/data' for a binary distribution, `/usr/local/var' for a source distribution). Look in the data directory for files with names of the form `host_name.err' and `host_name.log' where host_name is the name of your server host. Then check the last few lines of these files:

shell> tail host_name.err
shell> tail host_name.log

When the mysqld daemon starts up, it changes directory to the data directory. This is where it expects to write log files and the pid (process ID) file, and where it expects to find databases.

The data directory location is hardwired in when the distribution is compiled. However, if mysqld expects to find the data directory somewhere other than where it really is on your system, it will not work properly. If you have problems with incorrect paths, you can find out what options mysqld allows and what the default path settings are by invoking mysqld with the --help option. You can override the defaults by specifying the correct pathnames as command-line arguments to mysqld. (These options can be used with safe_mysqld as well.)

Normally you should need to tell mysqld only the base directory under which MySQL is installed. You can do this with the --basedir option. You can also use --help to check the effect of changing path options (note that --help must be the final option of the mysqld command). For example:

shell> EXECDIR/mysqld --basedir=/usr/local --help

Once you determine the path settings you want, start the server without the --help option.

If you get the following error, it means that some other program (or another mysqld server) is already using the TCP/IP port or socket mysqld is trying to use:

Can't start server: Bind on TCP/IP port: Address already in use
  or
Can't start server : Bind on unix socket...

Use ps to make sure that you don't have another mysqld server running. If you can't find another server running, you can try to execute the command telnet your-host-name tcp-ip-port-number and press RETURN a couple of times. If you don't get a error message like telnet: Unable to connect to remote host: Connection refused, something is using the TCP/IP port mysqld is trying to use. See section 4.15.1 Problems running mysql_install_db, and section 19.3 Running multiple MySQL servers on the same machine.

The safe_mysqld script is written so that it normally is able to start a server that was installed from either a source or a binary version of MySQL, even if these install the server in slightly different locations. safe_mysqld expects one of these conditions to be true:

Since safe_mysqld will try to find the server and databases relative to its own working directory, you can install a binary distribution of MySQL anywhere, as long as you start safe_mysqld from the MySQL installation directory:

shell> cd mysql_installation_directory
shell> bin/safe_mysqld &

If safe_mysqld fails, even when invoked from the MySQL installation directory, you can modify it to use the path to mysqld and the pathname options that are correct for your system. Note that if you upgrade MySQL in the future, your modified version of safe_mysqld will be overwritten, so you should make a copy of your edited version that you can reinstall.

If mysqld is currently running, you can find out what path settings it is using by executing this command:

shell> mysqladmin variables

or

shell> mysqladmin -h 'your-host-name' variables

If safe_mysqld starts the server but you can't connect to it, you should make sure you have an entry in `/etc/hosts' that looks like this:

127.0.0.1       localhost

This problem occurs only on systems that don't have a working thread library and for which MySQL must be configured to use MIT-pthreads.

4.15.3 Starting and stopping MySQL automatically

The mysql.server script can be used to start or stop the server, by invoking it with start or stop arguments:

shell> mysql.server start
shell> mysql.server stop

mysql.server can be found in the `share/mysql' directory under the MySQL installation directory, or in the `support-files' directory of the MySQL source tree.

Before mysql.server starts the server, it changes directory to the MySQL installation directory, then invokes safe_mysqld. You might need to edit mysql.server if you have a binary distribution that you've installed in a non-standard location. Modify it to cd into the proper directory before it runs safe_mysqld. If you want the server to run as some specific user, you can change the mysql_daemon_user=root line to use another user. You can also modify mysql.server to pass other options to safe_mysqld.

mysql.server stop brings down server by sending a signal to it. You can take down the server manually by executing mysqladmin shutdown.

You might want to add these start and stop commands to the appropriate places in your `/etc/rc*' files when you start using MySQL for production applications. Note that if you modify mysql.server, then if you upgrade MySQL sometime, your modified version will be overwritten, so you should make a copy of your edited version that you can reinstall.

If your system uses `/etc/rc.local' to start external scripts, you should append the following to it:

/bin/sh -c 'cd /usr/local/mysql ; ./bin/safe_mysqld &'

You can also add options or mysql.server in a global `/etc/my.cnf' file. A typical `/etc/my.cnf' file might look like this:

[mysqld]
datadir=/usr/local/mysql/var
socket=/tmp/mysqld.sock
port=3306

[mysql.server]
user=mysql
basedir=/usr/local/mysql

The mysql.server script uses the following variables: user, datadir, basedir, bindir and pid-file.

See section 4.15.4 Option files.

4.15.4 Option files

MySQL 3.22 can read default startup options for the server and for clients from option files.

MySQL reads default options from the following files on Unix:

Filename Purpose
/etc/my.cnf Global options
DATADIR/my.cnf Server-specific options
~/.my.cnf User-specific options

DATADIR is the MySQL data directory (typically `/usr/local/mysql/data' for a binary installation, or `/usr/local/var' for a source installation). Note that this is the directory that was specified at configuration time, not the one specified with --datadir when mysqld starts up! (--datadir has no effect on where the server looks for option files, because it looks for them before it processes any command-line arguments.)

MySQL reads default options from the following files on Win32:

Filename Purpose
windows-system-directory\my.ini
C:\my.cnf Global options
C:\mysql\data\my.cnf Server-specific options

Note that you on Win32 should specify all paths with / instead of \. If you use \, you need to specify this twice, as \ is the escape character in MySQL.

MySQL tries to read option files in the order listed above. If multiple option files exist, an option specified in a file read later takes precedence over the same option specified in a file read earlier. Options specified on the command line take precedence over options specified in any option file. Some options can be specified using environment variables. Options specified on the command line or in option files take precedence over environment variable values.

The following programs support option files: mysql, mysqladmin, mysqld, mysqldump, mysqlimport, mysql.server, isamchk and pack_isam.

You can use option files to specify any long option that a program supports! Run the program with --help to get a list of available options.

An option file can contain lines of the following forms:

#comment
Comment lines starts with `#' or `;'. Empty lines are ignored.
[group]
group is the name of the program or group for which you want to set options. After a group line, any option or set-variable lines apply to the named group until the end of the option file or another group line is given.
option
This is equivalent to --option on the command line.
option=value
This is equivalent to --option=value on the command line.
set-variable = variable=value
This is equivalent to --set-variable variable=value on the command line. This syntax must be used to set a mysqld variable.

The client group allows you to specify options that apply to all MySQL clients (not mysqld). This is the perfect group to use to specify the password you use to connect to the server. (But make sure the option file is readable and writable only to yourself.)

Note that for options and values, all leading and trailing blanks are automatically deleted. You may use the escape sequences `\b', `\t', `\n', `\r', `\\' and `\s' in your value string (`\s' == blank).

Here is a typical global option file:

[client]
port=3306
socket=/tmp/mysql.sock

[mysqld]
port=3306
socket=/tmp/mysql.sock
set-variable = key_buffer=16M
set-variable = max_allowed_packet=1M

[mysqldump]
quick

Here is typical user option file:

[client]
# The following password will be sent to all standard MySQL clients
password=my_password

[mysql]
no-auto-rehash

If you have a source distribution, you will find a sample configuration file named `my-example.cnf' in the `support-files' directory. If you have a binary distribution, look in the `DIR/share/mysql' directory, where DIR is the pathname to the MySQL installation directory (typically `/usr/local/mysql'). You can copy `my-example.cnf' to your home directory (rename the copy to `.my.cnf') to experiment with.

To tell a MySQL program not to read any option files, specify --no-defaults as the first option on the command line. This MUST be the first option or it will have no effect! If you want to check which options are used, you can give the option --print-defaults as the first option.

If you want to force the use of a specific config file, you can use the option --defaults-file=full-path-to-default-file. If you do this, only the specified file will be read.

Note for developers: Option file handling is implemented simply by processing all matching options (i.e., options in the appropriate group) before any command line arguments. This works nicely for programs that use the last instance of an option that is specified multiple times. If you have an old program that handles multiply-specified options this way but doesn't read option files, you need add only two lines to give it that capability. Check the source code of any of the standard MySQL clients to see how to do this.

4.16 Is there anything special to do when upgrading/downgrading MySQL?

You can always move the MySQL form and data files between different versions on the same architecture as long as you have the same base version of MySQL. The current base version is 3. If you change the character set by recompiling MySQL (which may also change the sort order), you must run isamchk -r -q on all tables. Otherwise your indexes may not be ordered correctly.

If you are paranoid and/or afraid of new versions, you can always rename your old mysqld to something like mysqld-'old-version-number'. If your new mysqld then does something unexpected, you can simply shut it down and restart with your old mysqld!

When you do an upgrade you should also backup your old databases, of course. Sometimes it's good to be a little paranoid!

After an upgrade, if you experience problems with recompiled client programs, like Commands out of sync or unexpected core dumps, you probably have used an old header or library file when compiling your programs. In this case you should check the date for your `mysql.h' file and `libmysqlclient.a' library to verify that they are from the new MySQL distribution. If not, please recompile your programs!

If you get some problems that the new mysqld server doesn't want to start or that you can't connect without a password, check that you don't have some old `my.cnf' file from your old installation! You can check this with: program-name --print-defaults. If this outputs anything other than the program name, you have a active my.cnf file that will may affect things!

It is a good idea to rebuild and reinstall the Msql-Mysql-modules distribution whenever you install a new release of MySQL, particularly if you notice symptoms such as all your DBI scripts dumping core after you upgrade MySQL.

4.16.1 Upgrading from a 3.22 version to 3.23

MySQL 3.23 supports tables of the new MyISAM type and the old NISAM type. You don't have to convert your old tables to use these with 3.23. By default, all new tables will be created with type MyISAM (unless you start mysqld with the --default-table-type=isam option. You can change an ISAM table to a MyISAM table with ALTER TABLE or the Perl script mysql_convert_table_format.

3.22 and 3.21 clients will work without any problems with a 3.23 server.

The following lists what you have to watch out for when upgrading to 3.23:

4.16.2 Upgrading from a 3.21 version to 3.22

Nothing that affects compatibility has changed between 3.21 and 3.22. The only pitfall is that new tables that are created with DATE type columns will use the new way to store the date. You can't access these new fields from an old version of mysqld.

After installing MySQL 3.22, you should start the new server and then run the mysql_fix_privilege_tables script. This will add the new privileges that you need to use the GRANT command. If you forget this, you will get Access denied when you try to use ALTER TABLE, CREATE INDEX or DROP INDEX. If your MySQL root user requires a password, you should give this as an argument to mysql_fix_privilege_tables.

The C API interface to mysql_real_connect() has changed. If you have an old client program that calls this function, you must place a 0 for the new db argument (or recode the client to send the db element for faster connections). You must also call mysql_init() before calling mysql_real_connect()! This change was done to allow the new mysql_options() function to save options in the MYSQL handler structure.

4.16.3 Upgrading from a 3.20 version to 3.21

If you are running a version older than 3.20.28 and want to switch to 3.21.x, you need to do the following:

You can start the mysqld 3.21 server with safe_mysqld --old-protocol to use it with clients from the 3.20 distribution. In this case, the new client function mysql_errno() will not return any server error, only CR_UNKNOWN_ERROR, (but it works for client errors) and the server uses the old password() checking rather than the new one.

If you are NOT using the --old-protocol option to mysqld, you will need to make the following changes:

MySQL 3.20.28 and above can handle the new user table format without affecting clients. If you have a MySQL version earlier than 3.20.28, passwords will no longer work with it if you convert the user table. So to be safe, you should first upgrade to at least 3.20.28 and then upgrade to 3.21.x.

The new client code works with a 3.20.x mysqld server, so if you experience problems with 3.21.x, you can use the old 3.20.x server without having to recompile the clients again.

If you are not using the --old-protocol option to mysqld, old clients will issue the error message:

ERROR: Protocol mismatch. Server Version = 10 Client Version = 9

The new Perl DBI/DBD interface also supports the old mysqlperl interface. The only change you have to make if you use mysqlperl is to change the arguments to the connect() function. The new arguments are: host, database, user, password (the user and password arguments have changed places). See section 20.5.2 The DBI interface.

The following changes may affect queries in old applications:

4.16.4 Upgrading to another architecture

If you are using MySQL 3.23, you can copy the .frm, the .MYI and the .MYD files between different architectures that support the same floating point format. (MySQL takes care of any byte swapping issues).

The MySQL data `*.ISD' and the index files `*.ISM' files) are architecture-dependent and in some case OS-dependent. If you want to move your applications to another machine that has a different architecture or OS than your current machine, you should not try to move a database by simply copying the files to the other machine. Use mysqldump instead.

By default, mysqldump will create a file full of SQL statements. You can then transfer the file to the other machine and feed it as input to the mysql client.

Try mysqldump --help to see what options are available. If you are moving the data to a newer version of MySQL, you should use mysqldump --opt with the newer version to get a fast, compact dump.

The easiest (although not the fastest) way to move a database between two machines is to run the following commands on the machine on which the database is located:

shell> mysqladmin -h 'other hostname' create db_name
shell> mysqldump --opt db_name \
        | mysql -h 'other hostname' db_name

If you want to copy a database from a remote machine over a slow network, you can use:

shell> mysqladmin create db_name
shell> mysqldump -h 'other hostname' --opt --compress db_name \
        | mysql db_name

You can also store the result in a file, then transfer the file to the target machine and load the file into the database there. For example, you can dump a database to a file on the source machine like this:

shell> mysqldump --quick db_name | gzip > db_name.contents.gz

(The file created in this example is compressed.) Transfer the file containing the database contents to the target machine and run these commands there:

shell> mysqladmin create db_name
shell> gunzip < db_name.contents.gz | mysql db_name

You can also use mysqldump and mysqlimport to accomplish the database transfer. For big tables, this is much faster than simply using mysqldump. In the commands shown below, DUMPDIR represents the full pathname of the directory you use to store the output from mysqldump.

First, create the directory for the output files and dump the database:

shell> mkdir DUMPDIR
shell> mysqldump --tab=DUMPDIR db_name

Then transfer the files in the DUMPDIR directory to some corresponding directory on the target machine and load the files into MySQL there:

shell> mysqladmin create db_name           # create database
shell> cat DUMPDIR/*.sql | mysql db_name   # create tables in database
shell> mysqlimport db_name DUMPDIR/*.txt   # load data into tables

Also, don't forget to copy the mysql database, since that's where the grant tables (user, db, host) are stored. You may have to run commands as the MySQL root user on the new machine until you have the mysql database in place.

After you import the mysql database on the new machine, execute mysqladmin flush-privileges so that the server reloads the grant table information.

5 How standards-compatible is MySQL?

5.1 MySQL extensions to ANSI SQL92

MySQL includes some extensions that you probably will not find in other SQL databases. Be warned that if you use them, your code will not be portable to other SQL servers. In some cases, you can write code that includes MySQL extensions, but is still portable, by using comments of the form /*! ... */. In this case, MySQL will parse and execute the code within the comment as it would any other MySQL statement, but other SQL servers will ignore the extensions. For example:

SELECT /*! STRAIGHT_JOIN */ col_name FROM table1,table2 WHERE ...

If you add a version number after the '!', the syntax will only be executed if the MySQL version is equal or newer than the used version number:

CREATE /*!32302 TEMPORARY */ TABLE (a int);

The above means that if you have 3.23.02 or newer, then MySQL will use the TEMPORARY keyword.

MySQL extensions are listed below:

5.2 MySQL differences compared to ANSI SQL92

We try to make MySQL follow the ANSI SQL standard and the ODBC SQL standard, but in some cases MySQL does some things differently:

5.3 Functionality missing from MySQL

The following functionality is missing in the current version of MySQL. For a prioritized list indicating when new extensions may be added to MySQL, you should consult the online MySQL TODO list. That is the latest version of the TODO list in this manual. See section F List of things we want to add to MySQL in the future (The TODO).

5.3.1 Sub-selects

The following will not yet work in MySQL:

SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);
SELECT * FROM table1 WHERE id NOT IN (SELECT id FROM table2);

However, in many cases you can rewrite the query without a sub select:

SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id;
SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id where table2.id IS NULL

For more complicated sub queries you can create temporary tables to hold the sub query.

MySQL only supports INSERT ... SELECT ... and REPLACE ... SELECT ... Independent sub-selects will be probably be available in 3.24.0. You can now use the function IN() in other contexts, however.

5.3.2 SELECT INTO TABLE

MySQL doesn't yet support the Oracle SQL extension: SELECT ... INTO TABLE .... MySQL supports instead the ANSI SQL syntax INSERT INTO ... SELECT ..., which is basically the same thing.

Alternatively, you can use SELECT INTO OUTFILE... or CREATE TABLE ... SELECT to solve your problem.

5.3.3 Transactions

Transactions are not supported. MySQL shortly will support atomic operations, which are like transactions without rollback. With atomic operations, you can execute a group of INSERT/SELECT/whatever commands and be guaranteed that no other thread will interfere. In this context, you won't usually need rollback. Currently, you can prevent interference from other threads by using the LOCK TABLES and UNLOCK TABLES commands. See section 7.23 LOCK TABLES/UNLOCK TABLES syntax.

5.3.4 Stored procedures and triggers

A stored procedure is a set of SQL commands that can be compiled and stored in the server. Once this has been done, clients don't need to keep reissuing the entire query but can refer to the stored procedure. This provides better performance because the query has to be parsed only once and less information needs to be sent between the server and the client. You can also raise the conceptual level by having libraries of functions in the server.

A trigger is a stored procedure that is invoked when a particular event occurs. For example, you can install a stored procedure that is triggered each time a record is deleted from a transaction table and that automatically deletes the corresponding customer from a customer table when all his transactions are deleted.

The planned update language will be able to handle stored procedures, but without triggers. Triggers usually slow down everything, even queries for which they are not needed.

To see when MySQL might get stored procedures, see section F List of things we want to add to MySQL in the future (The TODO).

5.3.5 Foreign Keys

Note that foreign keys in SQL are not used to join tables, but are used mostly for checking referential integrity. If you want to get results from multiple tables from a SELECT statement, you do this by joining tables!

SELECT * from table1,table2 where table1.id = table2.id;

See section 7.12 JOIN syntax. See section 8.3.5 Using foreign keys

The FOREIGN KEY syntax in MySQL exists only for compatibility with other SQL vendors' CREATE TABLE commands; it doesn't do anything. The FOREIGN KEY syntax without ON DELETE ... is mostly used for documentation purposes. Some ODBC applications may use this to produce automatic WHERE clauses, but this is usually easy to override. FOREIGN KEY is sometimes used as a constraint check, but this check is unnecessary in practice if rows are inserted into the tables in the right order. MySQL only supports these clauses because some applications require them to exist (regardless of whether or not they work!).

In MySQL, you can work around the problem of ON DELETE ... not being implemented by adding the appropriate DELETE statement to an application when you delete records from a table that has a foreign key. In practice this is as quick (in some cases quicker) and much more portable than using foreign keys.

In the near future we will extend the FOREIGN KEY implementation so that at least the information will be saved in the table specification file and may be retrieved by mysqldump and ODBC.

5.3.5.1 Reasons NOT to use foreign keys

There are so many problems with FOREIGN KEYs that we don't know where to start:

The only nice aspect of FOREIGN KEY is that it gives ODBC and some other client programs the ability to see how a table is connected and to use this to show connection diagrams and to help in building applicatons.

MySQL will soon store FOREIGN KEY definitions so that a client can ask for and receive an answer how the original connection was made. The current `.frm' file format does not have any place for it.

5.3.6 Views

MySQL doesn't support views, but this is on the TODO.

5.3.7 `--' as the start of a comment

Some other SQL databases use `--' to start comments. MySQL has `#' as the start comment character, even if the mysql command line tool removes all lines that start with `--'. You can also use the C comment style /* this is a comment */ with MySQL. See section 7.28 Comment syntax.

MySQL 3.23.3 and above supports the `--' comment style only if the comment is followed by a space. This is because this degenerate comment style has caused many problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for !payment!:

UPDATE tbl_name SET credit=credit-!payment!

What do you think will happen when the value of payment is negative?

Because 1--1 is legal in SQL, we think it is terrible that `--' means start comment.

In MySQL 3.23 you can however use: 1-- This is a comment

The following discussing only concerns you if you are running an MySQL version earlier than 3.23:

If you have a SQL program in a text file that contains `--' comments you should use:

shell> replace " --" " #" < text-file-with-funny-comments.sql \
         | mysql database

instead of the usual:

shell> mysql database < text-file-with-funny-comments.sql

You can also edit the command file "in place" to change the `--' comments to `#' comments:

shell> replace " --" " #" -- text-file-with-funny-comments.sql

Change them back with this command:

shell> replace " #" " --" -- text-file-with-funny-comments.sql

5.4 What standards does MySQL follow?

Entry level SQL92. ODBC level 0-2.

5.5 How to cope without COMMIT/ROLLBACK

MySQL doesn't support COMMIT-ROLLBACK. The problem is that handling COMMIT-ROLLBACK efficiently would require a completely different table layout than MySQL uses today. MySQL would also need extra threads that do automatic cleanups on the tables and the disk usage would be much higher. This would make MySQL about 2-4 times slower than it is today. MySQL is much faster than almost all other SQL databases (typically at least 2-3 times faster). One of the reasons for this is the lack of COMMIT-ROLLBACK.

For the moment, we are much more for implementing the SQL server language (something like stored procedures). With this you would very seldom really need COMMIT-ROLLBACK. This would also give much better performance.

Loops that need transactions normally can be coded with the help of LOCK TABLES, and you don't need cursors when you can update records on the fly.

We have transactions and cursors on the TODO but not quite prioritized. If we implement these, it will be as an option to CREATE TABLE. That means that COMMIT-ROLLBACK will work only on those tables, so that a speed penalty will be imposed on those table only.

We at TcX have a greater need for a real fast database than a 100% general database. Whenever we find a way to implement these features without any speed loss, we will probably do it. For the moment, there are many more important things to do. Check the TODO for how we prioritize things at the moment. (Customers with higher levels of support can alter this, so things may be reprioritized.)

The current problem is actually ROLLBACK. Without ROLLBACK, you can do any kind of COMMIT action with LOCK TABLES. To support ROLLBACK, MySQL would have to be changed to store all old records that were updated and revert everything back to the starting point if ROLLBACK was issued. For simple cases, this isn't that hard to do (the current isamlog could be used for this purpose), but it would be much more difficult to implement ROLLBACK for ALTER/DROP/CREATE TABLE.

To avoid using ROLLBACK, you can use the following strategy:

  1. Use LOCK TABLES ... to lock all the tables you want to access.
  2. Test conditions.
  3. Update if everything is okay.
  4. Use UNLOCK TABLES to release your locks.

This is usually a much faster method than using transactions with possible ROLLBACKs, although not always. The only situation this solution doesn't handle is when someone kills the threads in the middle of an update. In this case, all locks will be released but some of the updates may not have been executed.

You can also use functions to update records in a single operation. You can get a very efficient application by using the following techniques:

For example, when we are doing updates to some customer information, we update only the customer data that have changed and test only that none of the changed data, or data that depend on the changed data, have changed compared to the original row. The test for changed data is done with the WHERE clause in the UPDATE statement. If the record wasn't updated, we give the client a message: "Some of the data you have changed have been changed by another user". Then we show the old row versus the new row in a window, so the user can decide which version of the customer record he should use.

This gives us something that is similar to "column locking" but is actually even better, because we only update some of the columns, using values that are relative to their current values. This means that typical UPDATE statements look something like these:

UPDATE tablename SET pay_back=pay_back+'relative change';

UPDATE customer
  SET
    customer_date='current_date',
    address='new address',
    phone='new phone',
    money_he_owes_us=money_he_owes_us+'new_money'
  WHERE
    customer_id=id AND address='old address' AND phone='old phone';

As you can see, this is very efficient and works even if another client has changed the values in the pay_back or money_he_owes_us columns.

In many cases, users have wanted ROLLBACK and/or LOCK TABLES for the purpose of managing unique identifiers for some tables. This can be handled much more efficiently by using an AUTO_INCREMENT column and either the SQL function LAST_INSERT_ID() or the C API function mysql_insert_id(). See section 20.4.29 mysql_insert_id().

At TcX, we have never had any need for row-level locking because we have always been able to code around it. Some cases really need row locking, but they are very few. If you want row-level locking, you can use a flag column in the table and do something like this:

UPDATE tbl_name SET row_flag=1 WHERE id=ID;

MySQL returns 1 for the number of affected rows if the row was found and row_flag wasn't already 1 in the original row.

You can think of it as MySQL changed the above query to:

UPDATE tbl_name SET row_flag=1 WHERE id=ID and row_flag <> 1;

6 The MySQL access privilege system

MySQL has an advanced but non-standard security/privilege system. This section describes how it works.

6.1 What the privilege system does

The primary function of the MySQL privilege system is to authenticate a user connecting from a given host, and to associate that user with select, insert, update and delete privileges on a database.

Additional functionality includes the ability to have an anonymous user and to grant privileges for MySQL-specific functions such as LOAD DATA INFILE and administrative operations.

6.2 MySQL user names and passwords

There are several distinctions between the way user names and passwords are used by MySQL, and the way they are used by Unix or Windows:

6.3 Connecting to the MySQL server

MySQL client programs generally require that you specify connection parameters when you want to access a MySQL server: the host you want to connect to, your user name and your password. For example, the mysql client can be started like this (optional arguments are enclosed between `[' and `]'):

shell> mysql [-h host_name] [-u user_name] [-pyour_pass]

Alternate forms of the -h, -u and -p options are --host=host_name, --user=user_name and --password=your_pass. Note that there is no space between -p or --password= and the password following it.

Note: Specifing a password on the command line is not secure! Any user on your system may then find out your password by typing a command like: ps auxww. See section 4.15.4 Option files.

mysql uses default values for connection parameters that are missing from the command line:

Thus, for a Unix user joe, the following commands are equivalent:

shell> mysql -h localhost -u joe
shell> mysql -h localhost
shell> mysql -u joe
shell> mysql

Other MySQL clients behave similarly.

On Unix systems, you can specify different default values to be used when you make a connection, so that you need not enter them on the command line each time you invoke a client program. This can be done in a couple of ways:

If connection parameters are specified in multiple ways, values specified on the command line take precedence over values specified in configuration files and environment variables, and values in configuration files take precedence over values in environment variables.

6.4 Keeping your password secure

It is inadvisable to specify your password in a way that exposes it to discovery by other users. The methods you can use to specify your password when you run client programs are listed below, along with an assessment of the risks of each method:

All in all, the safest methods are to have the client program prompt for the password or to specify the password in a properly-protected `.my.cnf' file.

6.5 Privileges provided by MySQL

Privilege information is stored in the user, db, host, tables_priv and columns_priv tables in the mysql database (that is, in the database named mysql). The MySQL server reads the contents of these tables when it starts up and under the circumstances indicated in section 6.9 When privilege changes take effect.

The names used in this manual to refer to the privileges provided by MySQL are shown below, along with the table column name associated with each privilege in the grant tables and the context in which the privilege applies:

Privilege Column Context
select Select_priv tables
insert Insert_priv tables
update Update_priv tables
delete Delete_priv tables
index Index_priv tables
alter Alter_priv tables
create Create_priv databases, tables or indexes
drop Drop_priv databases or tables
grant Grant_priv databases or tables
reload Reload_priv server administration
shutdown Shutdown_priv server administration
process Process_priv server administration
file File_priv file access on server

The select, insert, update and delete privileges allow you to perform operations on rows in existing tables in a database.

SELECT statements require the select privilege only if they actually retrieve rows from a table. You can execute certain SELECT statements even without permission to access any of the databases on the server. For example, you could use the mysql client as a simple calculator:

mysql> SELECT 1+1;
mysql> SELECT PI()*2;

The index privilege allows you to create or drop (remove) indexes.

The alter privilege allows you to use ALTER TABLE.

The create and drop privileges allow you to create new databases and tables, or to drop (remove) existing databases and tables.

Note that if you grant the drop privilege for the mysql database to a user, that user can drop the database in which the MySQL access privileges are stored!

The grant privilege allows you to give to other users those privileges you yourself possess.

The file privilege gives you permission to read and write files on the server using the LOAD DATA INFILE and SELECT ... INTO OUTFILE statements. Any user to whom this privilege is granted can read or write any file that the MySQL server can read or write.

The remaining privileges are used for administrative operations, which are performed using the mysqladmin program. The table below shows which mysqladmin commands each administrative privilege allows you to execute:

Privilege Commands permitted to privilege holders
reload reload, refresh, flush-privileges, flush-hosts, flush-logs, flush-tables
shutdown shutdown
process processlist, kill

The reload command tells the server to reread the grant tables. The refresh command flushes all tables and opens and closes the log files. flush-privileges is a synonym for reload. The other flush-* commands perform functions similar to refresh but are more limited in scope, and may be preferable in some instances. For example, if you want to flush just the log files, flush-logs is a better choice than refresh.

The shutdown command shuts down the server.

The processlist command displays information about the threads executing within the server. The kill command kills server threads. You can always display or kill your own threads, but you need the process privilege to display or kill threads initiated by other users.

It is a good idea in general to grant privileges only to those users who need them, but you should exercise particular caution in granting certain privileges:

There are some things that you cannot do with the MySQL privilege system:

6.6 How the privilege system works

The MySQL privilege system ensures that all users may do exactly the things that they are supposed to be allowed to do. When you connect to a MySQL server, your identity is determined by the host from which you connect and the user name you specify. The system grants privileges according to your identity and what you want to do.

MySQL considers both your hostname and user name in identifying you because there is little reason to assume that a given user name belongs to the same person everywhere on the Internet. For example, the user bill who connects from whitehouse.gov need not be the same person as the user bill who connects from microsoft.com. MySQL handles this by allowing you to distinguish users on different hosts that happen to have the same name: you can grant bill one set of privileges for connections from whitehouse.gov, and a different set of privileges for connections from microsoft.com.

MySQL access control involves two stages:

The server uses the user, db and host tables in the mysql database at both stages of access control. The fields in these grant tables are shown below:

Table name user db host
Scope fields Host Host Host
User Db Db
Password User
Privilege fields Select_priv Select_priv Select_priv
Insert_priv Insert_priv Insert_priv
Update_priv Update_priv Update_priv
Delete_priv Delete_priv Delete_priv
Index_priv Index_priv Index_priv
Alter_priv Alter_priv Alter_priv
Create_priv Create_priv Create_priv
Drop_priv Drop_priv Drop_priv
Grant_priv Grant_priv Grant_priv
Reload_priv
Shutdown_priv
Process_priv
File_priv

For the second stage of access control (request verification), the server may, if the request involves tables, additionally consult the tables_priv and columns_priv tables. The fields in these tables are shown below:

Table name tables_priv columns_priv
Scope fields Host Host
Db Db
User User
Table_name Table_name
Column_name
Privilege fields Table_priv Column_priv
Column_priv
Other fields Timestamp Timestamp
Grantor

Each grant table contains scope fields and privilege fields.

Scope fields determine the scope of each entry in the tables, i.e., the context in which the entry applies. For example, a user table entry with Host and User values of 'thomas.loc.gov' and 'bob' would be used for authenticating connections made to the server by bob from the host thomas.loc.gov. Similarly, a db table entry with Host, User and Db fields of 'thomas.loc.gov', 'bob' and 'reports' would be used when bob connects from the host thomas.loc.gov to access the reports database. The tables_priv and columns_priv tables contain scope fields indicating tables or table/column combinations to which each entry applies.

For access-checking purposes, comparisons of Host values are case insensitive. User, Password, Db and Table_name values are case sensitive. Column_name values are case insensitive in MySQL 3.22.12 or later.

Privilege fields indicate the privileges granted by a table entry, that is, what operations can be performed. The server combines the information in the various grant tables to form a complete description of a user's privileges. The rules used to do this are described in section 6.8 Access control, stage 2: Request verification.

Scope fields are strings, declared as shown below; the default value for each is the empty string:

Field name Type
Host CHAR(60)
User CHAR(16)
Password CHAR(16)
Db CHAR(64) (CHAR(60) for the tables_priv and columns_priv tables)

In the user, db and host tables, all privilege fields are declared as ENUM('N','Y') -- each can have a value of 'N' or 'Y', and the default value is 'N'.

In the tables_priv and columns_priv tables, the privilege fields are declared as SET fields:

Table name Field name Possible set elements
tables_priv Table_priv 'Select', 'Insert', 'Update', 'Delete', 'Create', 'Drop', 'Grant', 'References', 'Index', 'Alter'
tables_priv Column_priv 'Select', 'Insert', 'Update', 'References'
columns_priv Column_priv 'Select', 'Insert', 'Update', 'References'

Briefly, the server uses the grant tables like this:

Note that administrative privileges (reload, shutdown, etc.) are specified only in the user table. This is because administrative operations are operations on the server itself and are not database-specific, so there is no reason to list such privileges in the other grant tables. In fact, only the user table need be consulted to determine whether or not you can perform an administrative operation.

The file privilege is specified only in the user table, too. It is not an administrative privilege as such, but your ability to read or write files on the server host is independent of the database you are accessing.

The mysqld server reads the contents of the grant tables once, when it starts up. Changes to the grant tables take effect as indicated in section 6.9 When privilege changes take effect.

When you modify the contents of the grant tables, it is a good idea to make sure that your changes set up privileges the way you want. For help in diagnosing problems, see section 6.13 Causes of Access denied errors. For advice on security issues, section 6.14 How to make MySQL secure against crackers.

A useful diagnostic tool is the mysqlaccess script, which Yves Carlier has provided for the MySQL distribution. Invoke mysqlaccess with the --help option to find out how it works. Note that mysqlaccess checks access using only the user, db and host tables. It does not check table- or column-level privileges.

6.7 Access control, stage 1: Connection verification

When you attempt to connect to a MySQL server, the server accepts or rejects the connection based on your identity and whether or not you can verify your identity by supplying the correct password. If not, the server denies access to you completely. Otherwise, the server accepts the connection, then enters stage 2 and waits for requests.

Your identity is based on two pieces of information:

Identity checking is performed using the three user table scope fields (Host, User and Password). The server accepts the connection only if a user table entry matches your hostname and user name, and you supply the correct password.

Values in the user table scope fields may be specified as follows:

Non-blank Password values represent encrypted passwords. MySQL does not store passwords in plaintext form for anyone to see. Rather, the password supplied by a user who is attempting to connect is encrypted (using the PASSWORD() function) and compared to the already-encrypted version stored in the user table. If they match, the password is correct.

The examples below show how various combinations of Host and User values in user table entries apply to incoming connections:

Host value User value Connections matched by entry
'thomas.loc.gov' 'fred' fred, connecting from thomas.loc.gov
'thomas.loc.gov' " Any user, connecting from thomas.loc.gov
'%' 'fred' fred, connecting from any host
'%' " Any user, connecting from any host
'%.loc.gov' 'fred' fred, connecting from any host in the loc.gov domain
'x.y.%' 'fred' fred, connecting from x.y.net, x.y.com,x.y.edu, etc. (this is probably not useful)
'144.155.166.177' 'fred' fred, connecting from the host with IP address 144.155.166.177
'144.155.166.%' 'fred' fred, connecting from any host in the 144.155.166 class C subnet

Since you can use IP wildcard values in the Host field (e.g., '144.155.166.%' to match every host on a subnet), there is the possibility that someone might try to exploit this capability by naming a host 144.155.166.somewhere.com. To foil such attempts, MySQL disallows matching on hostnames that start with digits and a dot. Thus, if you have a host named something like 1.2.foo.com, its name will never match the Host column of the grant tables. Only an IP number can match an IP wildcard value.

An incoming connection may be matched by more than one entry in the user table. For example, a connection from thomas.loc.gov by fred would be matched by several of the entries just shown above. How does the server choose which entry to use if more than one matches? The server resolves this question by sorting the user table after reading it at startup time, then looking through the entries in sorted order when a user attempts to connect. The first matching entry is the one that is used.

user table sorting works as follows. Suppose the user table looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| %         | root     | ...
| %         | jeffrey  | ...
| localhost | root     | ...
| localhost |          | ...
+-----------+----------+-

When the server reads in the table, it orders the entries with the most-specific Host values first ('%' in the Host column means "any host" and is least specific). Entries with the same Host value are ordered with the most-specific User values first (a blank User value means "any user" and is least specific). The resulting sorted user table looks like this:

+-----------+----------+-
| Host      | User     | ...
+-----------+----------+-
| localhost | root     | ...
| localhost |          | ...
| %         | jeffrey  | ...
| %         | root     | ...
+-----------+----------+-

When a connection is attempted, the server looks through the sorted entries and uses the first match found. For a connection from localhost by jeffrey, the entries with 'localhost' in the Host column match first. Of those, the entry with the blank user name matches both the connecting hostname and user name. (The '%'/'jeffrey' entry would have matched, too, but it is not the first match in the table.)

Here is another example. Suppose the user table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| %              | jeffrey  | ...
| thomas.loc.gov |          | ...
+----------------+----------+-

The sorted table looks like this:

+----------------+----------+-
| Host           | User     | ...
+----------------+----------+-
| thomas.loc.gov |          | ...
| %              | jeffrey  | ...
+----------------+----------+-

A connection from thomas.loc.gov by jeffrey is matched by the first entry, whereas a connection from whitehouse.gov by jeffrey is matched by the second.

A common misconception is to think that for a given user name, all entries that explicitly name that user will be used first when the server attempts to find a match for the connection. This is simply not true. The previous example illustrates this, where a connection from thomas.loc.gov by jeffrey is first matched not by the entry containing 'jeffrey' as the User field value, but by the entry with no user name!

If you have problems connecting to the server, print out the user table and sort it by hand to see where the first match is being made.

6.8 Access control, stage 2: Request verification

Once you establish a connection, the server enters stage 2. For each request that comes in on the connection, the server checks whether you have sufficient privileges to perform it, based on the type of operation you wish to perform. This is where the privilege fields in the grant tables come into play. These privileges can come from any of the user, db, host, tables_priv or columns_priv tables. The grant tables are manipulated with GRANT and REVOKE commands. See section 7.25 GRANT and REVOKE syntax. (You may find it helpful to refer to section 6.6 How the privilege system works, which lists the fields present in each of the grant tables.)

The user table grants privileges that are assigned to you on a global basis and that apply no matter what the current database is. For example, if the user table grants you the delete privilege, you can delete rows from any database on the server host! In other words, user table privileges are superuser privileges. It is wise to grant privileges in the user table only to superusers such as server or database administrators. For other users, you should leave the privileges in the user table set to 'N' and grant privileges on a database-specific basis only, using the db and host tables.

The db and host tables grant database-specific privileges. Values in the scope fields may be specified as follows:

The db and host tables are read in and sorted when the server starts up (at the same time that it reads the user table). The db table is sorted on the Host, Db and User scope fields, and the host table is sorted on the Host and Db scope fields. As with the user table, sorting puts the most-specific values first and least-specific values last, and when the server looks for matching entries, it uses the first match that it finds.

The tables_priv and columns_priv tables grant table- and column-specific privileges. Values in the scope fields may be specified as follows:

The tables_priv and columns_priv tables are sorted on the Host, Db and User fields. This is similar to db table sorting, although since only the Host field may contain wildcards, the sorting is simpler.

The request verification process is described below. (If you are familiar with the access-checking source code, you will notice that the description here differs slightly from the algorithm used in the code. The description is equivalent to what the code actually does; it differs only to make the explanation simpler.)

For administrative requests (shutdown, reload, etc.), the server checks only the user table entry, since that is the only table that specifies administrative privileges. Access is granted if the entry allows the requested operation and denied otherwise. For example, if you want to execute mysqladmin shutdown but your user table entry doesn't grant the shutdown privilege to you, access is denied without even checking the db or host tables. (Since they contain no Shutdown_priv column, there is no need to do so.)

For database-related requests (insert, update, etc.), the server first checks the user's global (superuser) privileges by looking in the user table entry. If the entry allows the requested operation, access is granted. If the global privileges in the user table are insufficient, the server determines the user's database-specific privileges by checking the db and host tables:

  1. The server looks in the db table for a match on the Host, Db and User fields. Host and User are matched to the connecting user's hostname and MySQL user name. The Db field is matched to the database the user wants to access. If there is no entry for the Host and User, access is denied.
  2. If there is a matching db table entry and its Host field is not blank, that entry defines the user's database-specific privileges.
  3. If the matching db table entry's Host field is blank, it signifies that the host table enumerates which hosts should be allowed access to the database. In this case, a further lookup is done in the host table to find a match on the Host and Db fields. If no host table entry matches, access is denied. If there is a match, the user's database-specific privileges are computed as the intersection (not the union!) of the privileges in the db and host table entries, i.e., the privileges that are 'Y' in both entries. (This way you can grant general privileges in the db table entry and then selectively restrict them on a host-by-host basis using the host table entries.)

After determining the database-specific privileges granted by the db and host table entries, the server adds them to the global privileges granted by the user table. If the result allows the requested operation, access is granted. Otherwise, the server checks the user's table and column privileges in the tables_priv and columns_priv tables and adds those to the user's privileges. Access is allowed or denied based on the result.

Expressed in boolean terms, the preceding description of how a user's privileges are calculated may be summarized like this:

global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges

It may not be apparent why, if the global user entry privileges are initially found to be insufficient for the requested operation, the server adds those privileges to the database-, table- and column-specific privileges later. The reason is that a request might require more than one type of privilege. For example, if you execute an INSERT ... SELECT statement, you need both insert and select privileges. Your privileges might be such that the user table entry grants one privilege and the db table entry grants the other. In this case, you have the necessary privileges to perform the request, but the server cannot tell that from either table by itself; the privileges granted by both entries must be combined.

The host table can be used to maintain a list of "secure" servers. At TcX, the host table contains a list of all machines on the local network. These are granted all privileges.

You can also use the host table to indicate hosts that are not secure. Suppose you have a machine public.your.domain that is located in a public area that you do not consider secure. You can allow access to all hosts on your network except that machine by using host table entries like this:

+--------------------+----+-
| Host               | Db | ...
+--------------------+----+-
| public.your.domain | %  | ... (all privileges set to 'N')
| %.your.domain      | %  | ... (all privileges set to 'Y')
+--------------------+----+-

Naturally, you should always test your entries in the grant tables (e.g., using mysqlaccess) to make sure your access privileges are actually set up the way you think they are.

6.9 When privilege changes take effect

When mysqld starts, all grant table contents are read into memory and become effective at that point.

Modifications to the grant tables that you perform using GRANT, REVOKE, or SET PASSWORD are noticed by the server immediately.

If you modify the grant tables manually (using INSERT, UPDATE, etc.), you should execute a FLUSH PRIVILEGES statement or run mysqladmin flush-privileges to tell the server to reload the grant tables. Otherwise your changes will have no effect until you restart the server.

When the server notices that the grant tables have been changed, existing client connections are affected as follows:

Global privilege changes and password changes take effect the next time the client connects.

6.10 Setting up the initial MySQL privileges

After installing MySQL, you set up the initial access privileges by running scripts/mysql_install_db. See section 4.7.1 Quick installation overview. The scripts/mysql_install_db script starts up the mysqld server, then initializes the grant tables to contain the following set of privileges:

Note: The default privileges are different for Win32. See section 4.12.4 Running MySQL on Win32.

Since your installation is initially wide open, one of the first things you should do is specify a password for the MySQL root user. You can do this as follows (note that you specify the password using the PASSWORD() function):

shell> mysql -u root mysql
mysql> UPDATE user SET Password=PASSWORD('new_password')
           WHERE user='root';
mysql> FLUSH PRIVILEGES;

You can also use the SET PASSWORD statement:

shell> mysql -u root mysql
mysql> SET PASSWORD FOR root=PASSWORD('new_password');

Another way to set the password is by using the mysqladmin command:

shell> mysqladmin -u root password new_password

Note that if you update the password in the user table directly using the first method, you must tell the server to reread the grant tables (with FLUSH PRIVILEGES), since the change will go unnoticed otherwise.

Once the root password has been set, thereafter you must supply that password when you connect to the server as root.

You may wish to leave the root password blank so that you don't need to specify it while you perform additional setup or testing, but be sure to set it before using your installation for any real production work.

See the scripts/mysql_install_db script to see how it sets up the default privileges. You can use this as a basis to see how to add other users.

If you want the initial privileges to be different than those just described above, you can modify mysql_install_db before you run it.

To recreate the grant tables completely, remove all the `*.frm', `*.ISM' and `*.ISD' files in the directory containing the mysql database. (This is the directory named `mysql' under the database directory, which is listed when you run mysqld --help.) Then run the mysql_install_db script, possibly after editing it first to have the privileges you want.

NOTE: For MySQL versions before 3.22.10, you should NOT delete the `*.frm' files. If you accidentally do this, you should copy them back from your MySQL distribution before running mysql_install_db.

6.11 Adding new user privileges to MySQL

You can add users two different ways: by using GRANT statements or by manipulating the MySQL grant tables directly. The preferred method is to use GRANT statements, because they are more concise and less error-prone.

The examples below show how to use the mysql client to set up new users. These examples assume that privileges are set up according to the defaults described in the previous section. This means that to make changes, you must be on the same machine where mysqld is running, you must connect as the MySQL root user, and the root user must have the insert privilege for the mysql database and the reload administrative privilege. Also, if you have changed the root user password, you must specify it for the mysql commands below.

You can add new users by issuing GRANT statements:

shell> mysql --user=root mysql
mysql> GRANT ALL PRIVILEGES ON *.* TO monty@localhost
           IDENTIFIED BY 'something' WITH GRANT OPTION;
mysql> GRANT ALL PRIVILEGES ON *.* TO monty@"%"
           IDENTIFIED BY 'something' WITH GRANT OPTION;
mysql> GRANT RELOAD,PROCESS ON *.* TO admin@localhost;
mysql> GRANT USAGE ON *.* TO dummy@localhost;

These GRANT statements set up three new users:

monty
A full superuser who can connect to the server from anywhere, but who must use a password ('something' to do so. Note that we must issue GRANT statements for both monty@localhost and monty@"%". If we don't add the entry with localhost, the anonymous user entry for localhost that is created by mysql_install_db will take precedence when we connect from the local host, because it has a more specific Host field value and thuse comes earlier in the user table sort order.
admin
A user who can connect from localhost without a password and who is granted the reload and process administrative privileges. This allows the user to execute the mysqladmin reload, mysqladmin refresh and mysqladmin flush-* commands, as well as mysqladmin processlist . No database-related privileges are granted. They can be granted later by issuing additional GRANT statements.
dummy
A user who can connect without a password, but only from the local host. The global privileges are all set to 'N' -- the USAGE privilege type allows you to set up a user with no privileges. It is assumed that you will grant database-specific privileges later.

You can also add the same user access information directly by issuing INSERT statements and then telling the server to reload the grant tables:

shell> mysql --user=root mysql
mysql> INSERT INTO user VALUES('localhost','monty',PASSWORD('something'),
                'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y')
mysql> INSERT INTO user VALUES('%','monty',PASSWORD('something'),
                'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y')
mysql> INSERT INTO user SET Host='localhost',User='admin',
                 Reload_priv='Y', Process_priv='Y';
mysql> INSERT INTO user (Host,User,Password)
                        VALUES('localhost','dummy',");
mysql> FLUSH PRIVILEGES;

Depending on your MySQL version, you may have to use a different number of 'Y' values above (versions prior to 3.22.11 had fewer privilege columns). For the admin user, the more readable extended INSERT syntax that is available starting with 3.22.11 is used.

Note that to set up a superuser, you need only create a user table entry with the privilege fields set to 'Y'. No db or host table entries are necessary.

The privilege columns in the user table were not set explicitly in the last INSERT statement (for the dummy user), so those columns are assigned the default value of 'N'. This is the same thing that GRANT USAGE does.

The following example adds a user custom who can connect from hosts localhost, server.domain and whitehouse.gov. He wants to access the bankaccount database only from localhost, the expenses database only from whitehouse.gov and the customer database from all three hosts. He wants to use the password stupid from all three hosts.

To set up this user's privileges using GRANT statements, run these commands:

shell> mysql --user=root mysql
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
           ON bankaccount.*
           TO custom@localhost
           IDENTIFIED BY 'stupid';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
           ON expenses.*
           TO custom@whitehouse.gov
           IDENTIFIED BY 'stupid';
mysql> GRANT SELECT,INSERT,UPDATE,DELETE,CREATE,DROP
           ON customer.*
           TO custom@'%'
           IDENTIFIED BY 'stupid';

To set up the user's privileges by modifying the grant tables directly, run these commands (note the FLUSH PRIVILEGES at the end):

shell> mysql --user=root mysql
mysql> INSERT INTO user (Host,User,Password)
       VALUES('localhost','custom',PASSWORD('stupid'));
mysql> INSERT INTO user (Host,User,Password)
       VALUES('server.domain','custom',PASSWORD('stupid'));
mysql> INSERT INTO user (Host,User,Password)
       VALUES('whitehouse.gov','custom',PASSWORD('stupid'));
mysql> INSERT INTO db
       (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES
       ('localhost','bankaccount','custom','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
       (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES
       ('whitehouse.gov','expenses','custom','Y','Y','Y','Y','Y','Y');
mysql> INSERT INTO db
       (Host,Db,User,Select_priv,Insert_priv,Update_priv,Delete_priv,
        Create_priv,Drop_priv)
       VALUES('%','customer','custom','Y','Y','Y','Y','Y','Y');
mysql> FLUSH PRIVILEGES;

The first three INSERT statements add user table entries that allow user custom to connect from the various hosts with the given password, but grant no permissions to him (all privileges are set to the default value of 'N'). The next three INSERT statements add db table entries that grant privileges to custom for the bankaccount, expenses and customer databases, but only when accessed from the proper hosts. As usual, when the grant tables are modified directly, the server must be told to reload them (with FLUSH PRIVILEGES) so that the privilege changes take effect.

If you want to give a specific user access from any machine in a given domain, you can issue a GRANT statement like the following:

mysql> GRANT ...
           ON *.*
           TO myusername@"%.mydomainname.com"
           IDENTIFIED BY 'mypassword';

To do the same thing by modifying the grant tables directly, do this:

mysql> INSERT INTO user VALUES ('%.mydomainname.com', 'myusername',
           PASSWORD('mypassword'),...);
mysql> FLUSH PRIVILEGES;

You can also use xmysqladmin, mysql_webadmin and even xmysql to insert, change and update values in the grant tables. You can find these utilities at the MySQL Contrib directory.

6.12 How to set up passwords

The examples in the preceding sections illustrate an important principle: when you store a non-empty password using INSERT or UPDATE statements, you must use the PASSWORD() function to encrypt it. This is because the user table stores passwords in encrypted form, not as plaintext. If you forget that fact, you are likely to attempt to set passwords like this:

shell> mysql -u root mysql
mysql> INSERT INTO user (Host,User,Password)
       VALUES('%','jeffrey','biscuit');
mysql> FLUSH PRIVILEGES;

The result is that the plaintext value 'biscuit' is stored as the password in the user table. When the user jeffrey attempts to connect to the server using this password, the mysql client encrypts it with PASSWORD() and sends the result to the server. The server compares the value in the user table (which is the plaintext value 'biscuit') to the encrypted password (which is not 'biscuit'). The comparison fails and the server rejects the connection:

shell> mysql -u jeffrey -pbiscuit test
Access denied

Since passwords must be encrypted when they are inserted in the user table, the INSERT statement should have been specified like this instead:

mysql> INSERT INTO user (Host,User,Password)
       VALUES('%','jeffrey',PASSWORD('biscuit'));

You must also use the PASSWORD() function when you use SET PASSWORD statements:

mysql> SET PASSWORD FOR jeffrey@"%" = PASSWORD('biscuit');

If you set passwords using the GRANT ... IDENTIFIED BY statement or the mysqladmin password command, the PASSWORD() function is unnecessary. They both take care of encrypting the password for you, so you would specify a password of 'biscuit' like this:

mysql> GRANT USAGE ON *.* TO jeffrey@"%" IDENTIFIED BY 'biscuit';

or

shell> mysqladmin -u jeffrey password biscuit

Note: PASSWORD() does not perform password encryption in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, PASSWORD() will result in the same encrypted value as is stored in the Unix password file. See section 6.2 MySQL user names and passwords.

6.13 Causes of Access denied errors

If you encounter Access denied errors when you try to connect to the MySQL server, the list below indicates some courses of action you can take to correct the problem:

6.14 How to make MySQL secure against crackers

When you connect to a MySQL server, you should normally use a password. The password is not transmitted in clear text over the connection.

All other information is transferred as text that can be read by anyone that is able to watch the connection. If you are concerned about this, you can use the compressed protocol (in MySQL 3.22 and above) to make things much harder. To make things even more secure you should install ssh (see http://www.cs.hut.fi/ssh). With this, you can get an encrypted TCP/IP connection between a MySQL server and a MySQL client.

To make a MySQL system secure, you should strongly consider the following suggestions:

The following mysqld options affect security:

--secure
IP numbers returned by the gethostbyname() system call are checked to make sure they resolve back to the original hostname. This makes it harder for someone on the outside to get access by simulating another host. This option also adds some sanity checks of hostnames. The option is turned off by default in MySQL 3.21 since it sometimes takes a long time to perform backward resolutions. MySQL 3.22 caches hostnames and has this option enabled by default.
--skip-grant-tables
This option causes the server not to use the privilege system at all. This gives everyone full access to all databases! (You can tell a running server to start using the grant tables again by executing mysqladmin reload.)
--skip-name-resolve
Hostnames are not resolved. All Host column values in the grant tables must be IP numbers or localhost.
--skip-networking
Don't allow TCP/IP connections over the network. All connections to mysqld must be made via Unix sockets. This option is unsuitable for systems that use MIT-pthreads, because the MIT-pthreads package doesn't support Unix sockets.

7 MySQL language reference

7.1 Literals: how to write strings and numbers

7.1.1 Strings

A string is a sequence of characters, surrounded by either single quote (`'') or double quote (`"') characters. Examples:

'a string'
"another string"

Within a string, certain sequences have special meaning. Each of these sequences begins with a backslash (`\'), known as the escape character. MySQL recognizes the following escape sequences:

\0
An ASCII 0 (NUL) character.
\n
A newline character.
\t
A tab character.
\r
A carriage return character.
\b
A backspace character.
\'
A single quote (`'') character.
\"
A double quote (`"') character.
\\
A backslash (`\') character.
\%
A `%' character. This is used to search for literal instances of `%' in contexts where `%' would otherwise be interpreted as a wildcard character.
\_
A `_' character. This is used to search for literal instances of `_' in contexts where `_' would otherwise be interpreted as a wildcard character.

There are several ways to include quotes within a string:

The SELECT statements shown below demonstrate how quoting and escaping work:

mysql> SELECT 'hello', '"hello"', '""hello""', 'hel"lo', '\'hello';
+-------+---------+-----------+--------+--------+
| hello | "hello" | ""hello"" | hel'lo | 'hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "hello", "'hello'", ""hello"", "hel""lo", "\"hello";
+-------+---------+-----------+--------+--------+
| hello | 'hello' | "hello" | hel"lo | "hello |
+-------+---------+-----------+--------+--------+

mysql> SELECT "This\nIs\nFour\nlines";
+--------------------+
| This
Is
Four
lines |
+--------------------+

If you want to insert binary data into a BLOB column, the following characters must be represented by escape sequences:

NUL
ASCII 0. You should represent this by `\0' (a backslash and an ASCII `0' character).
\
ASCII 92, backslash. Represent this by `\\'.
'
ASCII 39, single quote. Represent this by `\''.
"
ASCII 34, double quote. Represent this by `\"'.

If you write C code, you can use the C API function mysql_escape_string() to escape characters for the INSERT statement. See section 20.3 C API function overview. In Perl, you can use the quote method of the DBI package to convert special characters to the proper escape sequences. See section 20.5.2 The DBI interface.

You should use an escape function on any string that might contain any of the special characters listed above!

7.1.2 Numbers

Integers are represented as a sequence of digits. Floats use `.' as a decimal separator. Either type of number may be preceded by `-' to indicate a negative value.

Examples of valid integers:

1221
0
-32

Examples of valid floating-point numbers:

294.42
-32032.6809e+10
148.00

An integer may be used in a floating-point context; it is interpreted as the equivalent floating-point number.

7.1.3 Hexadecimal values

MySQL supports hexadecimal values. In number context these acts like an integer (64 bit precision). In string context these acts like a binary string where each pair of hex digits is converted to a character.

mysql> SELECT 0xa+0
       -> 10
mysql> select 0x5061756c;
       -> Paul

Hexadecimal strings is often used by ODBC to give values for BLOB columns.

7.1.4 NULL values

The NULL value means "no data" and is different from values such as 0 for numeric types or the empty string for string types. See section 18.15 Problems with NULL values.

NULL may be represented by \N when using the text file import or export formats (LOAD DATA INFILE, SELECT ... INTO OUTFILE). See section 7.15 LOAD DATA INFILE syntax.

7.1.5 Database, table, index, column and alias names

Database, table, index, column and alias names all follow the same rules in MySQL:

In MySQL you can refer to a column using any of the following forms:

Column reference Meaning
col_name Column col_name from whichever table used in the query contains a column of that name
tbl_name.col_name Column col_name from table tbl_name of the current database
db_name.tbl_name.col_name Column col_name from table tbl_name of the database db_name. This form is available in MySQL 3.22 or later.

You need not specify a tbl_name or db_name.tbl_name prefix for a column reference in a statement unless the reference would be ambiguous. For example, suppose tables t1 and t2 each contain a column c, and you retrieve c in a SELECT statement that uses both t1 and t2. In this case, c is ambiguous because it is not unique among the tables used in the statement, so you must indicate which table you mean by writing t1.c or t2.c. Similarly, if you are retrieving from a table t in database db1 and from a table t in database db2, you must refer to columns in those tables as db1.t.col_name and db2.t.col_name.

The syntax .tbl_name means the table tbl_name in the current database. This syntax is accepted for ODBC compatibility, because some ODBC programs prefix table names with a `.' character.

7.1.5.1 Case sensitivity in names

In MySQL, databases and tables correspond to directories and files within those directories. Consequently, the case sensitivity of the underlying operating system determines the case sensitivity of database and table names. This means database and table names are case sensitive in Unix and case insensitive in Win32.

Note: Although database and table names are case insensitive for Win32, you should not refer to a given database or table using different cases within the same query. The following query would not work because it refers to a table both as my_table and as MY_TABLE:

SELECT * FROM my_table WHERE MY_TABLE.col=1;

Column names are case insensitive in all cases.

Aliases on tables are case sensitive. The following query would not work because it refers to the alias both as a and as A:

mysql> SELECT col_name FROM tbl_name AS a
           WHERE a.col_name = 1 OR A.col_name = 2;

Aliases on columns are case insensitive.

7.2 Column types

MySQL supports a number of column types, which may be grouped into three categories: numeric types, date and time types, and string (character) types. This section first gives an overview of the types available and summarizes the storage requirements for each column type, then provides a more detailed description of the properties of the types in each category. The overview is intentionally brief. The more detailed descriptions should be consulted for additional information about particular column types, such as the allowable formats in which you can specify values.

The column types supported by MySQL are listed below. The following code letters are used in the descriptions:

M
Indicates the maximum display size. The maximum legal display size is 255.
D
Applies to floating-point types and indicates the number of digits following the decimal point.

Square brackets (`[' and `]') indicate parts of type specifiers that are optional.

Note that if you specify ZEROFILL for a column, MySQL will automatically add the UNSIGNED attribute to the column.

TINYINT[(M)] [UNSIGNED] [ZEROFILL]
A very small integer. The signed range is -128 to 127. The unsigned range is 0 to 255.
SMALLINT[(M)] [UNSIGNED] [ZEROFILL]
A small integer. The signed range is -32768 to 32767. The unsigned range is 0 to 65535.
MEDIUMINT[(M)] [UNSIGNED] [ZEROFILL]
A medium-size integer. The signed range is -8388608 to 8388607. The unsigned range is 0 to 16777215.
INT[(M)] [UNSIGNED] [ZEROFILL]
A normal-size integer. The signed range is -2147483648 to 2147483647. The unsigned range is 0 to 4294967295.
INTEGER[(M)] [UNSIGNED] [ZEROFILL]
This is a synonym for INT.
BIGINT[(M)] [UNSIGNED] [ZEROFILL]
A large integer. The signed range is -9223372036854775808 to 9223372036854775807. The unsigned range is 0 to 18446744073709551615. Note that all arithmetic is done using signed BIGINT or DOUBLE values, so you shouldn't use unsigned big integers larger than 9223372036854775807 (63 bits) except with bit functions! Note that -, + and * will use BIGINT arithmetic when both arguments are INTEGER values! This means that if you multiply two big integers (or results from functions that return integers) you may get unexpected results if the result is larger than 9223372036854775807.
FLOAT(precision) [ZEROFILL]
A floating-point number. Cannot be unsigned. precision can be 4 or 8. FLOAT(4) is a single-precision number and FLOAT(8) is a double-precision number. These types are like the FLOAT and DOUBLE types described immediately below. FLOAT(4) and FLOAT(8) have the same ranges as the corresponding FLOAT and DOUBLE types, but their display size and number of decimals is undefined. In MySQL 3.23, this is a true floating point value. In earlier MySQL versions, FLOAT(precision) always has 2 decimals. This syntax is provided for ODBC compatibility.
FLOAT[(M,D)] [ZEROFILL]
A small (single-precision) floating-point number. Cannot be unsigned. Allowable values are -3.402823466E+38 to -1.175494351E-38, 0 and 1.175494351E-38 to 3.402823466E+38.
DOUBLE[(M,D)] [ZEROFILL]
A normal-size (double-precision) floating-point number. Cannot be unsigned. Allowable values are -1.7976931348623157E+308 to -2.2250738585072014E-308, 0 and 2.2250738585072014E-308 to 1.7976931348623157E+308.
DOUBLE PRECISION[(M,D)] [ZEROFILL]
REAL[(M,D)] [ZEROFILL]
These are synonyms for DOUBLE.
DECIMAL(M,D) [ZEROFILL]
An unpacked floating-point number. Cannot be unsigned. Behaves like a CHAR column: "unpacked" means the number is stored as a string, using one character for each digit of the value, the decimal point, and, for negative numbers, the `-' sign. If D is 0, values will have no decimal point or fractional part. The maximum range of DECIMAL values is the same as for DOUBLE, but the actual range for a given DECIMAL column may be constrained by the choice of M and D. In MySQL 3.23 the M argument no longer includes the sign or the decimal point. (This is according to ANSI SQL.)
NUMERIC(M,D) [ZEROFILL]
This is a synonym for DECIMAL.
DATE
A date. The supported range is '1000-01-01' to '9999-12-31'. MySQL displays DATE values in 'YYYY-MM-DD' format, but allows you to assign values to DATE columns using either strings or numbers.
DATETIME
A date and time combination. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. MySQL displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format, but allows you to assign values to DATETIME columns using either strings or numbers.
TIMESTAMP[(M)]
A timestamp. The range is '1970-01-01 00:00:00' to sometime in the year 2037. MySQL displays TIMESTAMP values in YYYYMMDDHHMMSS, YYMMDDHHMMSS, YYYYMMDD or YYMMDD format, depending on whether M is 14 (or missing), 12, 8 or 6, but allows you to assign values to TIMESTAMP columns using either strings or numbers. A TIMESTAMP column is useful for recording the date and time of an INSERT or UPDATE operation because it is automatically set to the date and time of the most recent operation if you don't give it a value yourself. You can also set it to the current date and time by assigning it a NULL value. See section 7.2.6 Date and time types.
TIME
A time. The range is '-838:59:59' to '838:59:59'. MySQL displays TIME values in 'HH:MM:SS' format, but allows you to assign values to TIME columns using either strings or numbers.
YEAR
A year. The allowable values are 1901 to 2155, and 0000. MySQL displays YEAR values in YYYY format, but allows you to assign values to YEAR columns using either strings or numbers. (The YEAR type is new in MySQL 3.22.)
CHAR(M) [BINARY]
A fixed-length string that is always right-padded with spaces to the specified length when stored. The range of M is 1 to 255 characters. Trailing spaces are removed when the value is retrieved. CHAR values are sorted and compared in case-insensitive fashion unless the BINARY keyword is given.
VARCHAR(M) [BINARY]
A variable-length string. Note: Trailing spaces are removed when the value is stored (this differs from the ANSI SQL specification). The range of M is 1 to 255 characters. VARCHAR values are sorted and compared in case-insensitive fashion unless the BINARY keyword is given. See section 7.6.1 Silent column specification changes.
TINYBLOB
TINYTEXT
A BLOB or TEXT column with a maximum length of 255 (2^8 - 1) characters. See section 7.6.1 Silent column specification changes.
BLOB
TEXT
A BLOB or TEXT column with a maximum length of 65535 (2^16 - 1) characters. See section 7.6.1 Silent column specification changes.
MEDIUMBLOB
MEDIUMTEXT
A BLOB or TEXT column with a maximum length of 16777215 (2^24 - 1) characters. See section 7.6.1 Silent column specification changes.
LONGBLOB
LONGTEXT
A BLOB or TEXT column with a maximum length of 4294967295 (2^32 - 1) characters. See section 7.6.1 Silent column specification changes.
ENUM('value1','value2',...)
An enumeration. A string object that can have only one value, chosen from the list of values 'value1', 'value2', ..., or NULL. An ENUM can have a maximum of 65535 distinct values.
SET('value1','value2',...)
A set. A string object that can have zero or more values, each of which must be chosen from the list of values 'value1', 'value2', ... A SET can have a maximum of 64 members.

7.2.1 Column type storage requirements

The storage requirements for each of the column types supported by MySQL are listed below by category.

7.2.2 Numeric types

Column type Storage required
TINYINT 1 byte
SMALLINT 2 bytes
MEDIUMINT 3 bytes
INT 4 bytes
INTEGER 4 bytes
BIGINT 8 bytes
FLOAT(4) 4 bytes
FLOAT(8) 8 bytes
FLOAT 4 bytes
DOUBLE 8 bytes
DOUBLE PRECISION 8 bytes
REAL 8 bytes
DECIMAL(M,D) M bytes (D+2, if M < D)
NUMERIC(M,D) M bytes (D+2, if M < D)

7.2.3 Date and time types

Column type Storage required
DATETIME 8 bytes
DATE 3 bytes
TIMESTAMP 4 bytes
TIME 3 bytes
YEAR 1 byte

7.2.4 String types

Column type Storage required
CHAR(M) M bytes, 1 <= M <= 255
VARCHAR(M) L+1 bytes, where L <= M and 1 <= M <= 255
TINYBLOB, TINYTEXT L+1 bytes, where L < 2^8
BLOB, TEXT L+2 bytes, where L < 2^16
MEDIUMBLOB, MEDIUMTEXT L+3 bytes, where L < 2^24
LONGBLOB, LONGTEXT L+4 bytes, where L < 2^32
ENUM('value1','value2',...) 1 or 2 bytes, depending on the number of enumeration values (65535 values maximum)
SET('value1','value2',...) 1, 2, 3, 4 or 8 bytes, depending on the number of set members (64 members maximum)

VARCHAR and the BLOB and TEXT types are variable-length types, for which the storage requirements depend on the actual length of column values (represented by L in the preceding table), rather than on the type's maximum possible size. For example, a VARCHAR(10) column can hold a string with a maximum length of 10 characters. The actual storage required is the length of the string (L), plus 1 byte to record the length of the string. For the string 'abcd', L is 4 and the storage requirement is 5 bytes.

The BLOB and TEXT types require 1, 2, 3 or 4 bytes to record the length of the column value, depending on the maximum possible length of the type.

If a table includes any variable-length column types, the record format will also be variable-length. Note that when a table is created, MySQL may under certain conditions change a column from a variable-length type to a fixed-length type, or vice-versa. See section 7.6.1 Silent column specification changes.

The size of an ENUM object is determined by the number of different enumeration values. 1 byte is used for enumerations with up to 255 possible values. 2 bytes are used for enumerations with up to 65535 values.

The size of a SET object is determined by the number of different set members. If the set size is N, the object occupies (N+7)/8 bytes, rounded up to 1, 2, 3, 4 or 8 bytes. A SET can have a maximum of 64 members.

7.2.5 Numeric types

All integer types can have an optional attribute UNSIGNED. Unsigned values can be used when you want to allow only positive numbers in a column and you need a little bigger numeric range for the column.

All numeric types can have an optional attribute ZEROFILL. Values for ZEROFILL columns are left-padded with zeroes up to the maximum display length when they are displayed. For example, for a column declared as INT(5) ZEROFILL, a value of 4 is retrieved as 00004.

When asked to store a value in a numeric column that is outside the column type's allowable range, MySQL clips the value to the appropriate endpoint of the range and stores the resulting value instead.

For example, the range of an INT column is -2147483648 to 2147483647. If you try to insert -9999999999 into an INT column, the value is clipped to the lower endpoint of the range, and -2147483648 is stored instead. Similarly, if you try to insert 9999999999, 2147483647 is stored instead.

If the INT column is UNSIGNED, the size of the column's range is the same but its endpoints shift up to 0 and 4294967295. If you try to store -9999999999 and 9999999999, the values stored in the column become 0 and 4294967296.

Conversions that occur due to clipping are reported as "warnings" for ALTER TABLE, LOAD DATA INFILE, UPDATE and multi-row INSERT statements.

The maximum display size (M) and number of decimals (D) are used for formatting and calculation of maximum column width.

MySQL will store any value that fits a column's storage type even if the value exceeds the display size. For example, an INT(4) column has a display size of 4. Suppose you insert a value which has more than 4 digits into the column, such as 12345. The display size is exceeded, but the allowable range of the INT type is not, so MySQL stores the actual value, 12345. When retrieving the value from the column, MySQL returns the actual value stored in the column.

The DECIMAL type is considered a numeric type (as is its synonym, NUMERIC), but such values are stored as strings. One character is used for each digit of the value, the decimal point (if D > 0) and the `-' sign (for negative numbers). If D is 0, DECIMAL and NUMERIC values contain no decimal point or fractional part.

The maximum range of DECIMAL values is the same as for DOUBLE, but the actual range for a given DECIMAL column may be constrained by the choice of M and D. For example, a type specification such as DECIMAL(4,2) indicates a maximum length of four characters with two digits after the decimal point. Due to the way the DECIMAL type is stored, this specification results in an allowable range of -.99 to 9.99, much less than the range of a DOUBLE.

To avoid some rounding problems, MySQL always rounds everything that it stores in any floating-point column to the number of decimals indicated by the column specification. Suppose you have a column type of FLOAT(8,2). The number of decimals is 2, so a value such as 2.333 is rounded to two decimals and stored as 2.33.

7.2.6 Date and time types

The date and time types are DATETIME, DATE, TIMESTAMP, TIME and YEAR. Each of these has a range of legal values, as well as a "zero" value that is used when you specify an illegal value.

Here are some general considerations to keep in mind when working with date and time types:

7.2.6.1 Y2K issues and date types

MySQL itself is Y2K-safe (see section 1.6 Year 2000 compliance), but input values presented to MySQL may not be. Any input containing 2-digit year values is ambiguous, since the century is unknown. Such values must be interpreted into 4-digit form since MySQL stores years internally using four digits.

For DATETIME, DATE, TIMESTAMP and YEAR types, MySQL interprets dates with ambiguous year values using the following rules:

Remember that these rules provide only reasonable guesses as to what your data mean. If the heuristics used by MySQL don't produce the correct values, you should provide unambiguous input containing 4-digit year values.

7.2.6.2 The DATETIME, DATE and TIMESTAMP types

The DATETIME, DATE and TIMESTAMP types are related. This section describes their characteristics, how they are similar and how they differ.

The DATETIME type is used when you need values that contain both date and time information. MySQL retrieves and displays DATETIME values in 'YYYY-MM-DD HH:MM:SS' format. The supported range is '1000-01-01 00:00:00' to '9999-12-31 23:59:59'. ("Supported" means that although earlier values might work, there is no guarantee that they will.)

The DATE type is used when you need only a date value, without a time part. MySQL retrieves and displays DATE values in 'YYYY-MM-DD' format. The supported range is '1000-01-01' to '9999-12-31'.

The TIMESTAMP column type provides a type that you can use to automatically mark INSERT or UPDATE operations with the current date and time. If you have multiple TIMESTAMP columns, only the first one is updated automatically.

Automatic updating of the first TIMESTAMP column occurs under any of the following conditions:

TIMESTAMP columns other than the first may also be set to the current date and time. Just set the column to NULL, or to NOW().

You can set any TIMESTAMP column to a value different than the current date and time by setting it explicitly to the desired value. This is true even for the first TIMESTAMP column. You can use this property if, for example, you want a TIMESTAMP to be set to the current date and time when you create a row, but not to be changed whenever the row is updated later:

On the other hand, you may find it just as easy to use a DATETIME column that you initialize to NOW() when the row is created and leave alone for subsequent updates.

TIMESTAMP values may range from the beginning of 1970 to sometime in the year 2037, with a resolution of one second. Values are displayed as numbers.

The format in which MySQL retrieves and displays TIMESTAMP values depends on the display size, as illustrated by the table below. The `full' TIMESTAMP format is 14 digits, but TIMESTAMP columns may be created with shorter display sizes:

Column type Display format
TIMESTAMP(14) YYYYMMDDHHMMSS
TIMESTAMP(12) YYMMDDHHMMSS
TIMESTAMP(10) YYMMDDHHMM
TIMESTAMP(8) YYYYMMDD
TIMESTAMP(6) YYMMDD
TIMESTAMP(4) YYMM
TIMESTAMP(2) YY

All TIMESTAMP columns have the same storage size, regardless of display size. The most common display sizes are 6, 8, 12, and 14. You can specify an arbitrary display size at table creation time, but values of 0 or greater than 14 are coerced to 14. Odd-valued sizes in the range from 1 to 13 are coerced to the next higher even number.

You can specify DATETIME, DATE and TIMESTAMP values using any of a common set of formats:

Illegal DATETIME, DATE or TIMESTAMP values are converted to the "zero" value of the appropriate type ('0000-00-00 00:00:00', '0000-00-00' or 00000000000000).

For values specified as strings that include date part delimiters, it is not necessary to specify two digits for month or day values that are less than 10. '1979-6-9' is the same as '1979-06-09'. Similarly, for values specified as strings that include time part delimiters, it is not necessary to specify two digits for hour, month or second values that are less than 10. '1979-10-30 1:2:3' is the same as '1979-10-30 01:02:03'.

Values specified as numbers should be 6, 8, 12 or 14 digits long. If the number is 8 or 14 digits long, it is assumed to be in YYYYMMDD or YYYYMMDDHHMMSS format and that the year is given by the first 4 digits. If the number is 6 or 12 digits long, it is assumed to be in YYMMDD or YYMMDDHHMMSS format and that the year is given by the first 2 digits. Numbers that are not one of these lengths are interpreted as though padded with leading zeros to the closest length.

Values specified as non-delimited strings are interpreted using their length as given. If the string is 8 or 14 characters long, the year is assumed to be given by the first 4 characters. Otherwise the year is assumed to be given by the first 2 characters. The string is interpreted from left to right to find year, month, day, hour, minute and second values, for as many parts as are present in the string. This means you should not use strings that have fewer than 6 characters. For example, if you specify '9903', thinking that will represent March, 1999, you will find that MySQL inserts a "zero" date into your table. This is because the year and month values are 99 and 03, but the day part is missing (zero), so the value is not a legal date.

TIMESTAMP columns store legal values using the full precision with which the value was specified, regardless of the display size. This has several implications:

You can to some extent assign values of one date type to an object of a different date type. However, there may be some alteration of the value or loss of information:

Be aware of certain pitfalls when specifying date values:

7.2.6.3 The TIME type

MySQL retrieves and displays TIME values in 'HH:MM:SS' format (or 'HHH:MM:SS' format for large hours values). TIME values may range from '-838:59:59' to '838:59:59'. The reason the hours part may be so large is that the TIME type may be used not only to represent a time of day (which must be less than 24 hours), but also elapsed time or a time interval between two events (which may be much greater than 24 hours, or even negative).

You can specify TIME values in a variety of formats:

For TIME values specified as strings that include a time part delimiter, it is not necessary to specify two digits for hours, minutes or seconds values that are less than 10. '8:3:2' is the same as '08:03:02'.

Be careful about assigning "short" TIME values to a TIME column. MySQL interprets values using the assumption that the rightmost digits represent seconds. (MySQL interprets TIME values as elapsed time, rather than as time of day.) For example, you might think of '11:12', '1112' and 1112 as meaning '11:12:00' (12 minutes after 11 o'clock), but MySQL interprets them as '00:11:12' (11 minutes, 12 seconds). Similarly, '12' and 12 are interpreted as '00:00:12'.

Values that lie outside the TIME range but are otherwise legal are clipped to the appropriate endpoint of the range. For example, '-850:00:00' and '850:00:00' are converted to '-838:59:59' and '838:59:59'.

Illegal TIME values are converted to '00:00:00'. Note that since '00:00:00' is itself a legal TIME value, there is no way to tell, from a value of '00:00:00' stored in a table, whether the original value was specified as '00:00:00' or whether it was illegal.

7.2.6.4 The YEAR type

The YEAR type is a 1-byte type used for representing years.

MySQL retrieves and displays YEAR values in YYYY format. The range is 1901 to 2155.

You can specify YEAR values in a variety of formats:

Illegal YEAR values are converted to 0000.

7.2.7 String types

The string types are CHAR, VARCHAR, BLOB, TEXT, ENUM and SET.

7.2.7.1 The CHAR and VARCHAR types

The CHAR and VARCHAR types are similar, but differ in the way they are stored and retrieved.

The length of a CHAR column is fixed to the length that you declare when you create the table. The length can be any value between 1 and 255. When CHAR values are stored, they are right-padded with spaces to the specified length. When CHAR values are retrieved, trailing spaces are removed.

Values in VARCHAR columns are variable-length strings. You can declare a VARCHAR column to be any length between 1 and 255, just as for CHAR columns. However, in contrast to CHAR, VARCHAR values are stored using only as many characters as are needed, plus one byte to record the length. Values are not padded; instead, trailing spaces are removed when values are stored. (This space removal differs from the ANSI SQL specification.)

If you assign a value to a CHAR or VARCHAR column that exceeds the column's maximum length, the value is truncated to fit.

The table below illustrates the differences between the two types of columns by showing the result of storing various string values into CHAR(4) and VARCHAR(4) columns:

Value CHAR(4) Storage required VARCHAR(4) Storage required
" ' ' 4 bytes " 1 byte
'ab' 'ab ' 4 bytes 'ab' 3 bytes
'abcd' 'abcd' 4 bytes 'abcd' 5 bytes
'abcdefgh' 'abcd' 4 bytes 'abcd' 5 bytes

The values retrieved from the CHAR(4) and VARCHAR(4) columns will be the same in each case, because trailing spaces are removed from CHAR columns upon retrieval.

Values in CHAR and VARCHAR columns are sorted and compared in case-insensitive fashion, unless the BINARY attribute was specified when the table was created. The BINARY attribute means that column values are sorted and compared in case-sensitive fashion according to the ASCII order of the machine where the MySQL server is running.

The BINARY attribute is "sticky". This means that if a column marked BINARY is used in an expression, the whole expression is compared as a BINARY value.

MySQL may silently change the type of a CHAR or VARCHAR column at table creation time. See section 7.6.1 Silent column specification changes.

7.2.7.2 The BLOB and TEXT types

A BLOB is a binary large object that can hold a variable amount of data. The four BLOB types TINYBLOB, BLOB, MEDIUMBLOB and LONGBLOB differ only in the maximum length of the values they can hold. See section 7.2.1 Column type storage requirements.

The four TEXT types TINYTEXT, TEXT, MEDIUMTEXT and LONGTEXT correspond to the four BLOB types and have the same maximum lengths and storage requirements. The only difference between BLOB and TEXT types is that sorting and comparison is performed in case-sensitive fashion for BLOB values and case-insensitive fashion for TEXT values. In other words, a TEXT is a case-insensitive BLOB.

If you assign a value to a BLOB or TEXT column that exceeds the column type's maximum length, the value is truncated to fit.

In most respects, you can regard a TEXT column as a VARCHAR column that can be as big as you like. Similarly, you can regard a BLOB column as a VARCHAR BINARY column. The differences are:

MyODBC defines BLOB values as LONGVARBINARY and TEXT values as LONGVARCHAR.

Because BLOB and TEXT values may be extremely long, you may run up against some constraints when using them:

Note that each BLOB or TEXT value is represented internally by a separately-allocated object. This is in contrast to all other column types, for which storage is allocated once per column when the table is opened.

7.2.7.3 The ENUM type

An ENUM is a string object whose value normally is chosen from a list of allowed values that are enumerated explicitly in the column specification at table creation time.

The value may also be the empty string ("") or NULL under certain circumstances:

Each enumeration value has an index:

For example, a column specified as ENUM("one", "two", "three") can have any of the values shown below. The index of each value is also shown:

Value Index
NULL NULL
"" 0
"one" 1
"two" 2
"three" 3

An enumeration can have a maximum of 65535 elements.

Lettercase is irrelevant when you assign values to an ENUM column. However, values retrieved from the column later have lettercase matching the values that were used to specify the allowable values at table creation time.

If you retrieve an ENUM in a numeric context, the column value's index is returned. If you store a number into an ENUM, the number is treated as an index, and the the value stored is the enumeration member with that index.

ENUM values are sorted according to the order in which the enumeration members were listed in the column specification. (In other words, ENUM values are sorted according to their index numbers.) For example, "a" sorts before "b" for ENUM("a", "b"), but "b" sorts before "a" for ENUM("b", "a"). The empty string sorts before non-empty strings, and NULL values sort before all other enumeration values.

If you want to get all possible values for an ENUM column, you should use: SHOW COLUMNS FROM table_name LIKE enum_column_name and parse the ENUM definition in the second column.

7.2.7.4 The SET type

A SET is a string object that can have zero or more values, each of which must be chosen from a list of allowed values specified when the table is created. SET column values that consist of multiple set members are specified with members separated by commas (`,'). A consequence of this is that SET member values cannot themselves contain commas.

For example, a column specified as SET("one", "two") NOT NULL can have any of these values:

""
"one"
"two"
"one,two"

A SET can have a maximum of 64 different members.

MySQL stores SET values numerically, with the low-order bit of the stored value corresponding to the first set member. If you retrieve a SET value in a numeric context, the value retrieved has bits set corresponding to the set members that make up the column value. If a number is stored into a SET column, the bits that are set in the binary representation of the number determine the set members in the column value. Suppose a column is specified as SET("a","b","c","d"). Then the members have the following bit values:

SET member Decimal value Binary value
a 1 0001
b 2 0010
c 4 0100
d 8 1000

If you assign a value of 9 to this column, that is 1001 in binary, so the first and fourth SET value members "a" and "d" are selected and the resulting value is "a,d".

For a value containing more than one SET element, it does not matter what order the elements are listed in when you insert the value. It also doesn't not matter how many times a given element is listed in the value. When the value is retrieved later, each element in the value will appear once, with elements listed according to the order in which they were specified at table creation time. For example, if a column is specified as SET("a","b","c","d"), then "a,d", "d,a" and "d,a,a,d,d" will all appear as "a,d" when retrieved.

SET values are sorted numerically. NULL values sort before non-NULL SET values.

Normally, you perform a SELECT on a SET column using the LIKE operator or the FIND_IN_SET() function:

mysql> SELECT * FROM tbl_name WHERE set_col LIKE '%value%';
mysql> SELECT * FROM tbl_name WHERE FIND_IN_SET('value',set_col)>0;

But the following will also work:

mysql> SELECT * FROM tbl_name WHERE set_col = 'val1,val2';
mysql> SELECT * FROM tbl_name WHERE set_col & 1;

The first of these statements looks for an exact match. The second looks for values containing the first set member.

If you want to get all possible values for an SET column, you should use: SHOW COLUMNS FROM table_name LIKE set_column_name and parse the SET definition in the second column.

7.2.8 Choosing the right type for a column

For the most efficient use of storage, try to use the most precise type in all cases. For example, if an integer column will be used for values in the range between 1 and 99999, MEDIUMINT UNSIGNED is the best type.

Accurate representation of monetary values is a common problem. In MySQL, you should use the DECIMAL type. This is stored as a string, so no loss of accuracy should occur. If accuracy is not too important, the DOUBLE type may also be good enough.

For high precision, you can always convert to a fixed-point type stored in a BIGINT. This allows you to do all calculations with integers and convert results back to floating-point values only when necessary.

See section 10.17 What are the different row formats? Or, when should VARCHAR/CHAR be used?.

7.2.9 Column indexes

All MySQL column types can be indexed except BLOB and TEXT types. Use of indexes on the relevant columns is the best way to improve the performance of SELECT operations.

A table may have up to 16 indexes. The maximum index length is 256 bytes, although this may be changed when compiling MySQL.

You cannot index a column that may contain NULL values, so indexed columns must be declared NOT NULL.

For CHAR and VARCHAR columns, you can index a prefix of a column. This is much faster and requires less disk space than indexing the whole column. The syntax to use in the CREATE TABLE statement to index a column prefix looks like this:

KEY index_name (col_name(length))

The example below creates an index for the first 10 characters of the name column:

mysql> CREATE TABLE test (
           name CHAR(200) NOT NULL,
           KEY index_name (name(10)));

7.2.10 Multiple-column indexes

MySQL can create indexes on multiple columns. An index may consist of up to 15 columns. (On CHAR and VARCHAR columns you can also use a prefix of the column as a part of an index).

A multiple-column index can be considered a sorted array containing values that are created by concatenating the values of the indexed columns.

MySQL uses multiple-column indexes in such a way that queries are fast when you specify a known quantity for the first column of the index in a WHERE clause, even if you don't specify values for the other columns.

Suppose a table is created using the following specification:

mysql> CREATE TABLE test (
           id INT NOT NULL,
           last_name CHAR(30) NOT NULL,
           first_name CHAR(30) NOT NULL,
           PRIMARY KEY (id),
           INDEX name (last_name,first_name));

Then the index name is an index over last_name and first_name. The index will be used for queries that specify values in a known range for last_name, or for both last_name and first_name. Therefore, the name index will be used in the following queries:

mysql> SELECT * FROM test WHERE last_name="Widenius";

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          AND first_name="Michael";

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          AND (first_name="Michael" OR first_name="Monty");

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          AND first_name >="M" AND first_name < "N";

However, the name index will NOT be used in the following queries:

mysql> SELECT * FROM test WHERE first_name="Michael";

mysql> SELECT * FROM test WHERE last_name="Widenius"
                          OR first_name="Michael";

For more information on the manner in which MySQL uses indexes to improve query performance, see section 10.4 How MySQL uses indexes.

7.2.11 Using column types from other database engines

To make it easier to use code written for SQL implementations from other vendors, MySQL maps column types as shown in the table below. These mappings make it easier to move table definitions from other database engines to MySQL:

Other vendor type MySQL type
BINARY(NUM) CHAR(NUM) BINARY
CHAR VARYING(NUM) VARCHAR(NUM)
FLOAT4 FLOAT
FLOAT8 DOUBLE
INT1 TINYINT
INT2 SMALLINT
INT3 MEDIUMINT
INT4 INT
INT8 BIGINT
LONG VARBINARY MEDIUMBLOB
LONG VARCHAR MEDIUMTEXT
MIDDLEINT MEDIUMINT
VARBINARY(NUM) VARCHAR(NUM) BINARY

Column type mapping occurs at table creation time. If you create a table with types used by other vendors and then issue a DESCRIBE tbl_name statement, MySQL reports the table structure using the equivalent MySQL types.

7.3 Functions for use in SELECT and WHERE clauses

A select_expression or where_definition in a SQL statement can consist of any expression using the functions described below.

An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression.

Note: There must be no whitespace between a function name and the parenthesis following it. This helps the MySQL parser distinguish between function calls and references to tables or columns that happen to have the same name as a function. Spaces around arguments are permitted, though.

For the sake of brevity, examples display the output from the mysql program in abbreviated form. So this:

mysql> select MOD(29,9);
1 rows in set (0.00 sec)

+-----------+
| mod(29,9) |
+-----------+
|         2 |
+-----------+

Is displayed like this:

mysql> select MOD(29,9);
        -> 2

7.3.1 Grouping functions

( ... )
Parentheses. Use these to force the order of evaluation in an expression.
mysql> select 1+2*3;
        -> 7
mysql> select (1+2)*3;
        -> 9

7.3.2 Normal arithmetic operations

The usual arithmetic operators are available. Note that in the case of -, + and *, the result is calculated with BIGINT (64-bit) precision if both arguments are integers!

+
Addition
mysql> select 3+5;
        -> 8
-
Subtraction
mysql> select 3-5;
        -> -2
*
Multiplication
mysql> select 3*5;
        -> 15
mysql> select 18014398509481984*18014398509481984.0;
        -> 324518553658426726783156020576256.0
mysql> select 18014398509481984*18014398509481984;
        -> 0
The result of the last expression is incorrect because the result of the integer multiplication exceeds the 64-bit range of BIGINT calculations.
/
Division
mysql> select 3/5;
        -> 0.60
Division by zero produces a NULL result:
mysql> select 102/(1-1);
        -> NULL
A division will be calculated with BIGINT arithmetic only if performed in a context where its result is converted to an integer!

7.3.3 Bit functions

MySQL uses BIGINT (64-bit) arithmetic for bit operations, so these operators have a maximum range of 64 bits.

|
Bitwise OR
mysql> select 29 | 15;
        -> 31
&
Bitwise AND
mysql> select 29 & 15;
        -> 13
<<
Shifts a longlong (BIGINT) number to the left.
mysql> select 1 << 2
        -> 4
>>
Shifts a longlong (BIGINT) number to the right.
mysql> select 4 >> 2
        -> 1
BIT_COUNT(N)
Returns the number of bits that are set in the argument N.
mysql> select BIT_COUNT(29);
        -> 4

7.3.4 Logical operations

All logical functions return 1 (TRUE) or 0 (FALSE).

NOT
!
Logical NOT. Returns 1 if the argument is 0, otherwise returns 0. Exception: NOT NULL returns NULL.
mysql> select NOT 1;
        -> 0
mysql> select NOT NULL;
        -> NULL
mysql> select ! (1+1);
        -> 0
mysql> select ! 1+1;
        -> 1
The last example returns 1 because the expression evaluates the same way as (!1)+1.
OR
||
Logical OR. Returns 1 if either argument is not 0 and not NULL.
mysql> select 1 || 0;
        -> 1
mysql> select 0 || 0;
        -> 0
mysql> select 1 || NULL;
        -> 1

AND
&&
Logical AND. Returns 0 if either argument is 0 or NULL, otherwise returns 1.
mysql> select 1 && NULL;
        -> 0
mysql> select 1 && 0;
        -> 0

7.3.5 Comparison operators

Comparison operations result in a value of 1 (TRUE), 0 (FALSE) or NULL. These functions work for both numbers and strings. Strings are automatically converted to numbers and numbers to strings as needed (as in Perl).

MySQL performs comparisons using the following rules:

By default, string comparisons are done in case-independent fashion using the current character set (ISO-8859-1 Latin1 by default, which also works excellently for English).

The examples below illustrate conversion of strings to numbers for comparison operations:

mysql> SELECT 1 > '6x';
         -> 0
mysql> SELECT 7 > '6x';
         -> 1
mysql> SELECT 0 > 'x6';
         -> 0
mysql> SELECT 0 = 'x6';
         -> 1
=
Equal
mysql> select 1 = 0;
        -> 0
mysql> select '0' = 0;
        -> 1
mysql> select '0.0' = 0;
        -> 1
mysql> select '0.01' = 0;
        -> 0
mysql> select '.01' = 0.01;
        -> 1
<>
!=
Not equal
mysql> select '.01' <> '0.01';
        -> 1
mysql> select .01 <> '0.01';
        -> 0
mysql> select 'zapp' <> 'zappp';
        -> 1
<=
Less than or equal
mysql> select 0.1 <= 2;
        -> 1
<
Less than
mysql> select 2 <= 2;
        -> 1
>=
Greater than or equal
mysql> select 2 >= 2;
        -> 1
>
Greater than
mysql> select 2 > 2;
        -> 0
<=>
Null safe equal
mysql> select 1 <=> 1, NULL <=> NULL, 1 <=> NULL;
        -> 1 1 0
expr BETWEEN min AND max
If expr is greater than or equal to min and expr is less than or equal to max, BETWEEN returns 1, otherwise it returns 0. This is equivalent to the expression (min <= expr AND expr <= max) if all the arguments are of the same type. The first argument (expr) determines how the comparison is performed. If expr is a string expression, a case-insensitive string comparison is done. If expr is a binary string, a case-sensitive string comparison is done. If expr is an integer expression, an integer comparison is done. Otherwise, a floating-point (real) comparison is done.
mysql> select 1 BETWEEN 2 AND 3;
        -> 0
mysql> select 'b' BETWEEN 'a' AND 'c';
        -> 1
mysql> select 2 BETWEEN 2 AND '3';
        -> 1
mysql> select 2 BETWEEN 2 AND 'x-3';
        -> 0
expr IN (value,...)
Returns 1 if expr is any of the values in the IN list, else returns 0. If all values are constants, then all values are evaluated according to the type of expr and sorted. The search for the item is then done using a binary search. This means IN is very quick if the IN value list consists entirely of constants. If expr is a case-sensitive string expression, the string comparison is performed in case-sensitive fashion.
mysql> select 2 IN (0,3,5,'wefwf');
        -> 0
mysql> select 'wefwf' IN (0,3,5,'wefwf');
        -> 1
expr NOT IN (value,...)
Same as NOT (expr IN (value,...)).
ISNULL(expr)
If expr is NULL, ISNULL() returns 1, otherwise it returns 0.
mysql> select ISNULL(1+1);
        -> 0
mysql> select ISNULL(1/0);
        -> 1
Note that a comparison of NULL values using = will always be false!
COALESCE(list)
Returns first non-NULL element in list.
mysql> select COALESCE(NULL,1);
        -> 1
mysql> select COALESCE(NULL,NULL,NULL);
        -> NULL
INTERVAL(N,N1,N2,N3,...)
Returns 0 if N < N1, 1 if N < N2 and so on. All arguments are treated as numbers. It is required that N1 < N2 < N3 < ... < Nn for this function to work correctly. This is because a binary search is used (very fast).
mysql> select INTERVAL(23, 1, 15, 17, 30, 44, 200);
        -> 3
mysql> select INTERVAL(10, 1, 10, 100, 1000);
        -> 2
mysql> select INTERVAL(22, 23, 30, 44, 200);
        -> 0

7.3.6 String comparison functions

Normally, if any expression in a string comparison is case sensitive, the comparison is performed in case-sensitive fashion.

expr LIKE pat [ESCAPE 'escape-char']
Pattern matching using SQL simple regular expression comparison. Returns 1 (TRUE) or 0 (FALSE). With LIKE you can use the following two wildcard characters:
% Matches any number of characters, even zero characters
_ Matches exactly one character
mysql> select 'David!' LIKE 'David_';
        -> 1
mysql> select 'David!' LIKE '%D%v%';
        -> 1
To test for literal instances of a wildcard character, precede the character with the escape character. If you don't specify the ESCAPE character, `\' is assumed:
\% Matches one % character
\_ Matches one _ character
mysql> select 'David!' LIKE 'David\_';
        -> 0
mysql> select 'David_' LIKE 'David\_';
        -> 1
To specify a different escape character, use the ESCAPE clause:
mysql> select 'David_' LIKE 'David|_' ESCAPE '|';
        -> 1
LIKE is allowed on numeric expressions! (This is a MySQL extension to the ANSI SQL LIKE.)
mysql> select 10 LIKE '1%';
        -> 1
Note: Because MySQL uses the C escape syntax in strings (e.g., `\n'), you must double any `\' that you use in your LIKE strings. For example, to search for `\n', specify it as `\\n'. To search for `\', specify it as `\\\\' (the backslashes are stripped once by the parser, and another time when the pattern match is done, leaving a single backslash to be matched).
expr NOT LIKE pat [ESCAPE 'escape-char']
Same as NOT (expr LIKE pat [ESCAPE 'escape-char']).
expr REGEXP pat
expr RLIKE pat
Performs a pattern match of a string expression expr against a pattern pat. The pattern can be an extended regular expression. See section H Description of MySQL regular expression syntax. Returns 1 if expr matches pat, otherwise returns 0. RLIKE is a synonym for REGEXP, provided for mSQL compatibility. Note: Because MySQL uses the C escape syntax in strings (e.g., `\n'), you must double any `\' that you use in your REGEXP strings.
mysql> select 'Monty!' REGEXP 'm%y%%';
        -> 0
mysql> select 'Monty!' REGEXP '.*';
        -> 1
mysql> select 'new*\n*line' REGEXP 'new\\*.\\*line';
        -> 1
REGEXP and RLIKE use the current character set (ISO-8859-1 Latin1 by default) when deciding the type of a character.
expr NOT REGEXP pat
expr NOT RLIKE pat
Same as NOT (expr REGEXP pat).
STRCMP(expr1,expr2)
STRCMP() returns 0 if the strings are the same, -1 if the first argument is smaller than the second according to the current sort order, and 1 otherwise.
mysql> select STRCMP('text', 'text2');
        -> -1
mysql> select STRCMP('text2', 'text');
        -> 1
mysql> select STRCMP('text', 'text');
        -> 0

7.3.7 Cast operators

BINARY
The BINARY operator casts the string following it to a binary string. This is an easy way to force a column comparison to be case independent even if the column isn't defined as BINARY or BLOB.
mysql> select "a" = "A";
        -> 1
mysql> select BINARY "a" = "A";
        -> 0
BINARY was introduced in MySQL 3.23.0

7.3.8 Control flow functions

IFNULL(expr1,expr2)
If expr1 is not NULL, IFNULL() returns expr1, else it returns expr2. IFNULL() returns a numeric or string value, depending on the context in which it is used.
mysql> select IFNULL(1,0);
        -> 1
mysql> select IFNULL(0,10);
        -> 0
mysql> select IFNULL(1/0,10);
        -> 10
mysql> select IFNULL(1/0,'yes');
        -> 'yes'
IF(expr1,expr2,expr3)
If expr1 is TRUE (expr1 <> 0 and expr1 <> NULL) then IF() returns expr2, else it returns expr3. IFNULL() returns a numeric or string value, depending on the context in which it is used.
mysql> select IF(1>2,2,3);
        -> 3
mysql> select IF(1<2,'yes','no');
        -> 'yes'
mysql> select IF(strcmp('test','test1'),'yes','no');
        -> 'no'
expr1 is evaluated as an integer value, which means that if you are testing floating-point or string values, you should do so using a comparison operation.
mysql> select IF(0.1,1,0);
        -> 0
mysql> select IF(0.1<>0,1,0);
        -> 1
In the first case above, IF(0.1) returns 0 because 0.1 is converted to an integer value, resulting in a test of IF(0). This may not be what you expect. In the second case, the comparison tests the original floating-point value to see whether it is non-zero. The result of the comparison is used as an integer.
CASE value WHEN [compare-value] THEN result [WHEN [compare-value] THEN result ...] [ELSE result] END
CASE WHEN [condition] THEN result [WHEN [condition] THEN result ...] [ELSE result] END
The first version returns the result where value=compare-value. The second version returns the result for the first condition which is true. If there was no matching result value, then the result after ELSE is returned. If there is no ELSE part then NULL is returned.
mysql> SELECT CASE 1 WHEN 1 THEN "one" WHEN 2 THEN "two" ELSE "more" END;
       -> "one"
mysql> SELECT CASE WHEN 1>0 THEN "true" ELSE "false" END;
       -> "true"
mysql> SELECT CASE BINARY "B" when "a" then 1 when "b" then 2 END;
       -> NULL

7.3.9 Mathematical functions

All mathematical functions return NULL in case of an error.

-
Unary minus. Changes the sign of the argument.
mysql> select - 2;
        -> -2
Note that if this operator is used with a BIGINT, the return value is a BIGINT! This means that you should avoid using - on integers that may have the value of -2^63!
ABS(X)
Returns the absolute value of X.
mysql> select ABS(2);
        -> 2
mysql> select ABS(-32);
        -> 32
This function is safe to use with BIGINT values.
SIGN(X)
Returns the sign of the argument as -1, 0 or 1, depending on whether X is negative, zero, or positive.
mysql> select SIGN(-32);
        -> -1
mysql> select SIGN(0);
        -> 0
mysql> select SIGN(234);
        -> 1
MOD(N,M)
%
Modulo (like the % operator in C). Returns the remainder of N divided by M.
mysql> select MOD(234, 10);
        -> 4
mysql> select 253 % 7;
        -> 1
mysql> select MOD(29,9);
        -> 2
This function is safe to use with BIGINT values.
FLOOR(X)
Returns the largest integer value not greater than X.
mysql> select FLOOR(1.23);
        -> 1
mysql> select FLOOR(-1.23);
        -> -2
Note that the return value is converted to a BIGINT!
CEILING(X)
Returns the smallest integer value not less than X.
mysql> select CEILING(1.23);
        -> 2
mysql> select CEILING(-1.23);
        -> -1
Note that the return value is converted to a BIGINT!
ROUND(X)
Returns the argument X, rounded to an integer.
mysql> select ROUND(-1.23);
        -> -1
mysql> select ROUND(-1.58);
        -> -2
mysql> select ROUND(1.58);
        -> 2
Note that the return value is converted to a BIGINT!
ROUND(X,D)
Returns the argument X, rounded to a number with D decimals. If D is 0, the result will have no decimal point or fractional part.
mysql> select ROUND(1.298, 1);
        -> 1.3
mysql> select ROUND(1.298, 0);
        -> 1
Note that the return value is converted to a BIGINT!
EXP(X)
Returns the value of e (the base of natural logarithms) raised to the power of X.
mysql> select EXP(2);
        -> 7.389056
mysql> select EXP(-2);
        -> 0.135335
LOG(X)
Returns the natural logarithm of X.
mysql> select LOG(2);
        -> 0.693147
mysql> select LOG(-2);
        -> NULL
If you want the log of a number X to some arbitary base B, use the formula LOG(X)/LOG(B).
LOG10(X)
Returns the base-10 logarithm of X.
mysql> select LOG10(2);
        -> 0.301030
mysql> select LOG10(100);
        -> 2.000000
mysql> select LOG10(-100);
        -> NULL
POW(X,Y)
POWER(X,Y)
Returns the value of X raised to the power of Y.
mysql> select POW(2,2);
        -> 4.000000
mysql> select POW(2,-2);
        -> 0.250000
SQRT(X)
Returns the non-negative square root of X.
mysql> select SQRT(4);
        -> 2.000000
mysql> select SQRT(20);
        -> 4.472136
PI()
Returns the value of PI.
mysql> select PI();
        -> 3.141593
COS(X)
Returns the cosine of X, where X is given in radians.
mysql> select COS(PI());
        -> -1.000000
SIN(X)
Returns the sine of X, where X is given in radians.
mysql> select SIN(PI());
        -> 0.000000
TAN(X)
Returns the tangent of X, where X is given in radians.
mysql> select TAN(PI()+1);
        -> 1.557408
ACOS(X)
Returns the arc cosine of X, that is, the value whose cosine is X. Returns NULL if X is not in the range -1 to 1.
mysql> select ACOS(1);
        -> 0.000000
mysql> select ACOS(1.0001);
        -> NULL
mysql> select ACOS(0);
        -> 1.570796
ASIN(X)
Returns the arc sine of X, that is, the value whose sine is X. Returns NULL if X is not in the range -1 to 1.
mysql> select ASIN(0.2);
        -> 0.201358
mysql> select ASIN('foo');
        -> 0.000000
ATAN(X)
Returns the arc tangent of X, that is, the value whose tangent is X.
mysql> select ATAN(2);
        -> 1.107149
mysql> select ATAN(-2);
        -> -1.107149
ATAN2(X,Y)
Returns the arc tangent of the two variables X and Y. It is similar to calculating the arc tangent of Y / X, except that the signs of both arguments are used to determine the quadrant of the result.
mysql> select ATAN(-2,2);
        -> -0.785398
mysql> select ATAN(PI(),0);
        -> 1.570796
COT(X)
Returns the cotangent of X.
mysql> select COT(12);
        -> -1.57267341
mysql> select COT(0);
        -> NULL
RAND()
RAND(N)
Returns a random floating-point value in the range 0 to 1.0. If an integer argument N is specified, it is used as the seed value.
mysql> select RAND();
        -> 0.5925
mysql> select RAND(20);
        -> 0.1811
mysql> select RAND(20);
        -> 0.1811
mysql> select RAND();
        -> 0.2079
mysql> select RAND();
        -> 0.7888
You can't use a column with RAND() values in an ORDER BY clause, because ORDER BY would evaluate the column multiple times. In MySQL 3.23, you can however do: SELECT * FROM table_name ORDER BY RAND() This is useful to get a random sample of a set SELECT * FROM table1,table2 WHERE a=b AND c<d ORDER BY RAND() LIMIT 1000.
LEAST(X,Y,...)
With two or more arguments, returns the smallest (minimum-valued) argument. The arguments are compared using the following rules:
mysql> select LEAST(2,0);
        -> 0
mysql> select LEAST(34.0,3.0,5.0,767.0);
        -> 3.0
mysql> select LEAST("B","A","C");
        -> "A"
In MySQL versions prior to 3.22.5, you can use MIN() instead of LEAST.
GREATEST(X,Y,...)
Returns the largest (maximum-valued) argument. The arguments are compared using the same rules as for LEAST.
mysql> select GREATEST(2,0);
        -> 2
mysql> select GREATEST(34.0,3.0,5.0,767.0);
        -> 767.0
mysql> select GREATEST("B","A","C");
        -> "C"
In MySQL versions prior to 3.22.5, you can use MAX() instead of GREATEST.
DEGREES(X)
Returns the argument X, converted from radians to degrees.
mysql> select DEGREES(PI());
        -> 180.000000
RADIANS(X)
Returns the argument X, converted from degrees to radians.
mysql> select RADIANS(90);
        -> 1.570796
TRUNCATE(X,D)
Returns the number X, truncated to D decimals.
mysql> select TRUNCATE(1.223,1);
        -> 1.2
mysql> select TRUNCATE(1.999,1);
        -> 1.9
mysql> select TRUNCATE(1.999,0);
        -> 1

7.3.10 String functions

String-valued functions return NULL if the length of the result would be greater than the max_allowed_packet server parameter. See section 10.1 Tuning server parameters.

For functions that operate on string positions, the first position is numbered 1.

ASCII(str)
Returns the ASCII code value of the leftmost character of the string str. Returns 0 if str is the empty string. Returns NULL if str is NULL.
mysql> select ASCII('2');
        -> 50
mysql> select ASCII(2);
        -> 50
mysql> select ASCII('dx');
        -> 100
CONV(N,from_base,to_base)
Converts numbers between different number bases. Returns a string representation of the number N, converted from base from_base to base to_base. Returns NULL if any argument is NULL. The argument N is interpreted as an integer, but may be specified as an integer or a string. The minimum base is 2 and the maximum base is 36. If to_base is a negative number, N is regarded as a signed number. Otherwise, N is treated as unsigned. CONV works with 64-bit precision.
mysql> select CONV("a",16,2);
        -> '1010'
mysql> select CONV("6E",18,8);
        -> '172'
mysql> select CONV(-17,10,-18);
        -> '-H'
mysql> select CONV(10+"10"+'10'+0xa,10,10);
        -> '40'
BIN(N)
Returns a string representation of the binary value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,2). Returns NULL if N is NULL.
mysql> select BIN(12);
        -> '1100'
OCT(N)
Returns a string representation of the octal value of N, where N is a longlong number. This is equivalent to CONV(N,10,8). Returns NULL if N is NULL.
mysql> select OCT(12);
        -> '14'
HEX(N)
Returns a string representation of the hexadecimal value of N, where N is a longlong (BIGINT) number. This is equivalent to CONV(N,10,16). Returns NULL if N is NULL.
mysql> select HEX(255);
        -> 'FF'
CHAR(N,...)
CHAR() interprets the arguments as integers and returns a string consisting of the characters given by the ASCII code values of those integers. NULL values are skipped.
mysql> select CHAR(77,121,83,81,'76');
        -> 'MySQL'
mysql> select CHAR(77,77.3,'77.3');
        -> 'MMM'
CONCAT(X,Y,...)
Returns the string that results from concatenating the arguments. Returns NULL if any argument is NULL. May have more than 2 arguments.
mysql> select CONCAT('My', 'S', 'QL');
        -> 'MySQL'
mysql> select CONCAT('My', NULL, 'QL');
        -> NULL
LENGTH(str)
OCTET_LENGTH(str)
CHAR_LENGTH(str)
CHARACTER_LENGTH(str)
Returns the length of the string str.
mysql> select LENGTH('text');
        -> 4
mysql> select OCTET_LENGTH('text');
        -> 4
LOCATE(substr,str)
POSITION(substr IN str)
Returns the position of the first occurrence of substring substr in string str. Returns 0 if substr is not in str.
mysql> select LOCATE('bar', 'foobarbar');
        -> 4
mysql> select LOCATE('xbar', 'foobar');
        -> 0
LOCATE(substr,str,pos)
Returns the position of the first occurrence of substring substr in string str, starting at position pos. Returns 0 if substr is not in str.
mysql> select LOCATE('bar', 'foobarbar',5);
        -> 7
INSTR(str,substr)
Returns the position of the first occurrence of substring substr in string str. This is the same as the two-argument form of LOCATE(), except that the arguments are swapped.
mysql> select INSTR('foobarbar', 'bar');
        -> 4
mysql> select INSTR('xbar', 'foobar');
        -> 0
LPAD(str,len,padstr)
Returns the string str, left-padded with the string padstr until str is len characters long.
mysql> select LPAD('hi',4,'??');
        -> '??hi'
RPAD(str,len,padstr)
Returns the string str, right-padded with the string padstr until str is len characters long.
mysql> select RPAD('hi',5,'?');
        -> 'hi???'
LEFT(str,len)
Returns the leftmost len characters from the string str.
mysql> select LEFT('foobarbar', 5);
        -> 'fooba'
RIGHT(str,len)
Returns the rightmost len characters from the string str.
mysql> select RIGHT('foobarbar', 4);
        -> 'rbar'
mysql> select SUBSTRING('foobarbar' FROM 4);
        -> 'rbar'
SUBSTRING(str,pos,len)
SUBSTRING(str FROM pos FOR len)
MID(str,pos,len)
Returns a substring len characters long from string str, starting at position pos. The variant form that uses FROM is ANSI SQL92 syntax.
mysql> select SUBSTRING('Quadratically',5,6);
        -> 'ratica'
SUBSTRING(str,pos)
SUBSTRING(str FROM pos)
Returns a substring from string str starting at position pos.
mysql> select SUBSTRING('Quadratically',5);
        -> 'ratically'
SUBSTRING_INDEX(str,delim,count)
Returns the substring from string str after count occurrences of the delimiter delim. If count is positive, everything to the left of the final delimiter (counting from the left) is returned. If count is negative, everything to the right of the final delimiter (counting from the right) is returned.
mysql> select SUBSTRING_INDEX('www.mysql.com', '.', 2);
        -> 'www.mysql'
mysql> select SUBSTRING_INDEX('www.mysql.com', '.', -2);
        -> 'mysql.com'
LTRIM(str)
Returns the string str with leading space characters removed.
mysql> select LTRIM('  barbar');
        -> 'barbar'
RTRIM(str)
Returns the string str with trailing space characters removed.
mysql> select RTRIM('barbar   ');
        -> 'barbar'
TRIM([[BOTH | LEADING | TRAILING] [remstr] FROM] str)
Returns the string str with all remstr prefixes and/or suffixes removed. If none of the specifiers BOTH, LEADING or TRAILING are given, BOTH is assumed. If remstr is not specified, spaces are removed.
mysql> select TRIM('  bar   ');
        -> 'bar'
mysql> select TRIM(LEADING 'x' FROM 'xxxbarxxx');
        -> 'barxxx'
mysql> select TRIM(BOTH 'x' FROM 'xxxbarxxx');
        -> 'bar'
mysql> select TRIM(TRAILING 'xyz' FROM 'barxxyz');
        -> 'barx'
SOUNDEX(str)
Returns a soundex string from str. Two strings that sound "about the same" should have identical soundex strings. A "standard" soundex string is 4 characters long, but the SOUNDEX() function returns an arbitrarily long string. You can use SUBSTRING() on the result to get a "standard" soundex string. All non-alphanumeric characters are ignored in the given string. All international alpha characters outside the A-Z range are treated as vowels.
mysql> select SOUNDEX('Hello');
        -> 'H400'
mysql> select SOUNDEX('Quadratically');
        -> 'Q36324'
SPACE(N)
Returns a string consisting of N space characters.
mysql> select SPACE(6);
        -> '      '
REPLACE(str,from_str,to_str)
Returns the string str with all all occurrences of the string from_str replaced by the string to_str.
mysql> select REPLACE('www.mysql.com', 'w', 'Ww');
        -> 'WwWwWw.mysql.com'
REPEAT(str,count)
Returns a string consisting of the string str repeated count times. If count <= 0, returns an empty string. Returns NULL if str or count are NULL.
mysql> select REPEAT('MySQL', 3);
        -> 'MySQLMySQLMySQL'
REVERSE(str)
Returns the string str with the order of the characters reversed.
mysql> select REVERSE('abc');
        -> 'cba'
INSERT(str,pos,len,newstr)
Returns the string str, with the substring beginning at position pos and len characters long replaced by the string newstr.
mysql> select INSERT('Quadratic', 3, 4, 'What');
        -> 'QuWhattic'
ELT(N,str1,str2,str3,...)
Returns str1 if N = 1, str2 if N = 2, and so on. Returns NULL if N is less than 1 or greater than the number of arguments. ELT() is the complement of FIELD().
mysql> select ELT(1, 'ej', 'Heja', 'hej', 'foo');
        -> 'ej'
mysql> select ELT(4, 'ej', 'Heja', 'hej', 'foo');
        -> 'foo'
FIELD(str,str1,str2,str3,...)
Returns the index of str in the str1, str2, str3, ... list. Returns 0 if str is not found. FIELD() is the complement of ELT().
mysql> select FIELD('ej', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 2
mysql> select FIELD('fo', 'Hej', 'ej', 'Heja', 'hej', 'foo');
        -> 0
FIND_IN_SET(str,strlist)
Returns a value 1 to N if the string str is in the list strlist consisting of N substrings. A string list is a string composed of substrings separated by `,' characters. If the first argument is a constant string and the second is a column of type SET, the FIND_IN_SET() function is optimized to use bit arithmetic! Returns 0 if str is not in strlist or if strlist is the empty string. Returns NULL if either argument is NULL. This function will not work properly if the first argument contains a `,'.
mysql> SELECT FIND_IN_SET('b','a,b,c,d');
        -> 2
MAKE_SET(bits,str1,str2,...)
Returns a set (a string containing substrings separated by `,' characters) consisting of the strings that have the corresponding bit in bits set. str1 corresponds to bit 0, str2 to bit 1, etc. NULL strings in str1, str2, ... are not appended to the result.
mysql> SELECT MAKE_SET(1,'a','b','c');
        -> 'a'
mysql> SELECT MAKE_SET(1 | 4,'hello','nice','world');
        -> 'hello,world'
mysql> SELECT MAKE_SET(0,'a','b','c');
        -> "
EXPORT_SET(bits,on,off,[separator,[number_of_bits]])
Returns a string where for every bit set in 'bit', you get a 'on' string and for every reset bit you get an 'off' string. Each string is separated with 'separator' (default ',') and only 'number_of_bits' (default 64) of 'bits' is used.
mysql> select EXPORT_SET(5,'Y','N',',',4)
        -> Y,N,Y,N 
LCASE(str)
LOWER(str)
Returns the string str with all characters changed to lowercase according to the current character set mapping (the default is ISO-8859-1 Latin1).
mysql> select LCASE('QUADRATICALLY');
        -> 'quadratically'
UCASE(str)
UPPER(str)
Returns the string str with all characters changed to uppercase according to the current character set mapping (the default is ISO-8859-1 Latin1).
mysql> select UCASE('Hej');
        -> 'HEJ'
LOAD_FILE(file_name)
Reads the file and returns the file contents as a string. The file must be on the server, you must specify the full pathname to the file, and you must have the file privilege. The file must be readable by all and be smaller than max_allowed_packet. If the file doesn't exist or can't be read due to one of the above reasons, the function returns NULL.
mysql> UPDATE table_name SET blob_column=LOAD_FILE("/tmp/picture") WHERE id=1;

There is no string function to convert a number to a char. There is no need for one, because MySQL automatically converts numbers to strings as necessary, and vice versa:

mysql> SELECT 1+"1";
        -> 2
mysql> SELECT CONCAT(2,' test');
        -> '2 test'

If a string function is given a binary string as an argument, the resulting string is also a binary string. A number converted to a string is treated as a binary string. This only affects comparisons.

7.3.11 Date and time functions

See section 7.2.6 Date and time types for a description of the range of values each type has, and the valid formats in which date and time values may be specified.

Here is an example that uses date functions. The query below selects all records with a date_col value from within the last 30 days:

mysql> SELECT something FROM table
           WHERE TO_DAYS(NOW()) - TO_DAYS(date_col) <= 30;
DAYOFWEEK(date)
Returns the weekday index for date (1 = Sunday, 2 = Monday, ... 7 = Saturday). These index values correspond to the ODBC standard.
mysql> select DAYOFWEEK('1998-02-03');
        -> 3
WEEKDAY(date)
Returns the weekday index for date (0 = Monday, 1 = Tuesday, ... 6 = Sunday).
mysql> select WEEKDAY('1997-10-04 22:23:00');
        -> 5
mysql> select WEEKDAY('1997-11-05');
        -> 2
DAYOFMONTH(date)
Returns the day of the month for date, in the range 1 to 31.
mysql> select DAYOFMONTH('1998-02-03');
        -> 3
DAYOFYEAR(date)
Returns the day of the year for date, in the range 1 to 366.
mysql> select DAYOFYEAR('1998-02-03');
        -> 34
MONTH(date)
Returns the month for date, in the range 1 to 12.
mysql> select MONTH('1998-02-03');
        -> 2
DAYNAME(date)
Returns the name of the weekday for date.
mysql> select DAYNAME("1998-02-05");
        -> 'Thursday'
MONTHNAME(date)
Returns the name of the month for date.
mysql> select MONTHNAME("1998-02-05");
        -> 'February'
QUARTER(date)
Returns the quarter of the year for date, in the range 1 to 4.
mysql> select QUARTER('98-04-01');
        -> 2
WEEK(date)
WEEK(date,first)
With a single argument, returns the week for date, in the range 0 to 52, for locations where Sunday is the first day of the week. The two-argument form of WEEK() allows you to specify whether the week starts on Sunday or Monday. The week starts on Sunday if the second argument is 0, on Monday if the second argument is 1.
mysql> select WEEK('1998-02-20');
        -> 7
mysql> select WEEK('1998-02-20',0);
        -> 7
mysql> select WEEK('1998-02-20',1);
        -> 8
YEAR(date)
Returns the year for date, in the range 1000 to 9999.
mysql> select YEAR('98-02-03');
        -> 1998
HOUR(time)
Returns the hour for time, in the range 0 to 23.
mysql> select HOUR('10:05:03');
        -> 10
MINUTE(time)
Returns the minute for time, in the range 0 to 59.
mysql> select MINUTE('98-02-03 10:05:03');
        -> 5
SECOND(time)
Returns the second for time, in the range 0 to 59.
mysql> select SECOND('10:05:03');
        -> 3
PERIOD_ADD(P,N)
Adds N months to period P (in the format YYMM or YYYYMM). Returns a value in the format YYYYMM. Note that the period argument P is not a date value.
mysql> select PERIOD_ADD(9801,2);
        -> 199803
PERIOD_DIFF(P1,P2)
Returns the number of months between periods P1 and P2. P1 and P2 should be in the format YYMM or YYYYMM. Note that the period arguments P1 and P2 are not date values.
mysql> select PERIOD_DIFF(9802,199703);
        -> 11
DATE_ADD(date,INTERVAL expr type)
DATE_SUB(date,INTERVAL expr type)
ADDDATE(date,INTERVAL expr type)
SUBDATE(date,INTERVAL expr type)
These functions perform date arithmetic. They are new for MySQL 3.22. ADDDATE() and SUBDATE() are synonyms for DATE_ADD() and DATE_SUB(). date is a DATETIME or DATE value specifying the starting date. expr is an expression specifying the interval value to be added or substracted from the starting date. expr is a string; it may start with a `-' for negative intervals. type is a keyword indicating how the expression should be interpreted. The EXTRACT() function returns the corresponding interval from the date. The following table shows how the type and expr arguments are related:
type value Meaning Expected expr format
SECOND Seconds SECONDS
MINUTE Minutes MINUTES
HOUR Hours HOURS
DAY Days DAYS
MONTH Months MONTHS
YEAR Years YEARS
MINUTE_SECOND Minutes and seconds "MINUTES:SECONDS"
HOUR_MINUTE Hours and minutes "HOURS:MINUTES"
DAY_HOUR Days and hours "DAYS HOURS"
YEAR_MONTH Years and months "YEARS-MONTHS"
HOUR_SECOND Hours, minutes, "HOURS:MINUTES:SECONDS"
DAY_MINUTE Days, hours, minutes "DAYS HOURS:MINUTES"
DAY_SECOND Days, hours, minutes, seconds "DAYS HOURS:MINUTES:SECONDS"
MySQL allows any non-numeric delimiter in the expr format. The ones shown in the table are the suggested delimiters. If the date argument is a DATE value and your calculations involve only YEAR, MONTH and DAY parts (that is, no time parts), the result is a DATE value. Otherwise the result is a DATETIME value.
mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
                       INTERVAL 1 SECOND);
        -> 1998-01-01 00:00:00
mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
                       INTERVAL 1 DAY);
        -> 1998-01-01 23:59:59
mysql> SELECT DATE_ADD("1997-12-31 23:59:59",
                       INTERVAL "1:1" MINUTE_SECOND);
        -> 1998-01-01 00:01:00
mysql> SELECT DATE_SUB("1998-01-01 00:00:00",
                       INTERVAL "1 1:1:1" DAY_SECOND);
        -> 1997-12-30 22:58:59
mysql> SELECT DATE_ADD("1998-01-01 00:00:00",
                       INTERVAL "-1 10" DAY_HOUR);
        -> 1997-12-30 14:00:00
mysql> SELECT DATE_SUB("1998-01-02", INTERVAL 31 DAY);
        -> 1997-12-02
mysql> SELECT EXTRACT(YEAR FROM "1999-07-02");
       -> 1999
mysql> SELECT EXTRACT(YEAR_MONTH FROM "1999-07-02 01:02:03");
       -> 199907
mysql> SELECT EXTRACT(DAY_MINUTE FROM "1999-07-02 01:02:03");
       -> 20102
If you specify an interval value that is too short (does not include all the interval parts that would be expected from the type keyword), MySQL assumes you have left out the leftmost parts of the interval value. For example, if you specify a type of DAY_SECOND, the value of expr is expected to have days, hours, minutes and seconds parts. If you specify a value like "1:10", MySQL assumes that the days and hours parts are missing and the value represents minutes and seconds. In other words, "1:10" DAY_SECOND is interpreted in such a way that it is equivalent to "1:10" MINUTE_SECOND. This is analogous to the way that MySQL interprets TIME values as representing elapsed time rather than as time of day. If you use incorrect dates, the result is NULL. If you add MONTH, YEAR_MONTH or YEAR and the resulting date has a day that is larger than the maximum day for the new month, the day is adjusted to the maximum days in the new month.
mysql> select DATE_ADD('1998-01-30', Interval 1 month);
        -> 1998-02-28
Note from the preceding example that the word INTERVAL and the type keyword are not case sensitive.
TO_DAYS(date)
Given a date date, returns a daynumber (the number of days since year 0).
mysql> select TO_DAYS(950501);
        -> 728779
mysql> select TO_DAYS('1997-10-07');
        -> 729669
TO_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582).
FROM_DAYS(N)
Given a daynumber N, returns a DATE value.
mysql> select FROM_DAYS(729669);
        -> '1997-10-07'
FROM_DAYS() is not intended for use with values that precede the advent of the Gregorian calendar (1582).
DATE_FORMAT(date,format)
Formats the date value according to the format string. The following specifiers may be used in the format string:
%M Month name (January..December)
%W Weekday name (Sunday..Saturday)
%D Day of the month with english suffix (1st, 2nd, 3rd, etc.)
%Y Year, numeric, 4 digits
%y Year, numeric, 2 digits
%a Abbreviated weekday name (Sun..Sat)
%d Day of the month, numeric (00..31)
%e Day of the month, numeric (0..31)
%m Month, numeric (01..12)
%c Month, numeric (1..12)
%b Abbreviated month name (Jan..Dec)
%j Day of year (001..366)
%H Hour (00..23)
%k Hour (0..23)
%h Hour (01..12)
%I Hour (01..12)
%l Hour (1..12)
%i Minutes, numeric (00..59)
%r Time, 12-hour (hh:mm:ss [AP]M)
%T Time, 24-hour (hh:mm:ss)
%S Seconds (00..59)
%s Seconds (00..59)
%p AM or PM
%w Day of the week (0=Sunday..6=Saturday)
%U Week (0..52), where Sunday is the first day of the week
%u Week (0..52), where Monday is the first day of the week
%% A literal `%'.
All other characters are just copied to the result without interpretation.
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%W %M %Y');
        -> 'Saturday October 1997'
mysql> select DATE_FORMAT('1997-10-04 22:23:00', '%H:%i:%s');
        -> '22:23:00'
mysql> select DATE_FORMAT('1997-10-04 22:23:00',
                          '%D %y %a %d %m %b %j');
        -> '4th 97 Sat 04 10 Oct 277'
mysql> select DATE_FORMAT('1997-10-04 22:23:00',
                          '%H %k %I %r %T %S %w');
        -> '22 22 10 10:23:00 PM 22:23:00 00 6'
As of MySQL 3.23, the % is required before a format specifier characters. In earlier versions of MySQL, % was optional.
TIME_FORMAT(time,format)
This is used like the DATE_FORMAT() function above, but the format string may contain only those format specifiers that handle hours, minutes and seconds. Other specifiers produce a NULL value or 0.
CURDATE()
CURRENT_DATE
Returns today's date as a value in 'YYYY-MM-DD' or YYYYMMDD format, depending on whether the function is used in a string or numeric context.
mysql> select CURDATE();
        -> '1997-12-15'
mysql> select CURDATE() + 0;
        -> 19971215
CURTIME()
CURRENT_TIME
Returns the current time as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context.
mysql> select CURTIME();
        -> '23:50:26'
mysql> select CURTIME() + 0;
        -> 235026
NOW()
SYSDATE()
CURRENT_TIMESTAMP
Returns the current date and time as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context.
mysql> select NOW();
        -> '1997-12-15 23:50:26'
mysql> select NOW() + 0;
        -> 19971215235026
UNIX_TIMESTAMP()
UNIX_TIMESTAMP(date)
If called with no argument, returns a Unix timestamp (seconds since '1970-01-01 00:00:00' GMT). If UNIX_TIMESTAMP() is called with a date argument, it returns the value of the argument as seconds since '1970-01-01 00:00:00' GMT. date may be a DATE string, a DATETIME string, a TIMESTAMP, or a number in the format YYMMDD or YYYYMMDD in local time.
mysql> select UNIX_TIMESTAMP();
        -> 882226357
mysql> select UNIX_TIMESTAMP('1997-10-04 22:23:00');
        -> 875996580
When UNIX_TIMESTAMP is used on a TIMESTAMP column, the function will receive the value directly, with no implicit "string-to-unix-timestamp" conversion.
FROM_UNIXTIME(unix_timestamp)
Returns a representation of the unix_timestamp argument as a value in 'YYYY-MM-DD HH:MM:SS' or YYYYMMDDHHMMSS format, depending on whether the function is used in a string or numeric context.
mysql> select FROM_UNIXTIME(875996580);
        -> '1997-10-04 22:23:00'
mysql> select FROM_UNIXTIME(875996580) + 0;
        -> 19971004222300
FROM_UNIXTIME(unix_timestamp,format)
Returns a string representation of the Unix timestamp, formatted according to the format string. format may contain the same specifiers as those listed in the entry for the DATE_FORMAT() function.
mysql> select FROM_UNIXTIME(UNIX_TIMESTAMP(),
                            '%Y %D %M %h:%i:%s %x');
        -> '1997 23rd December 03:43:30 x'
SEC_TO_TIME(seconds)
Returns the seconds argument, converted to hours, minutes and seconds, as a value in 'HH:MM:SS' or HHMMSS format, depending on whether the function is used in a string or numeric context.
mysql> select SEC_TO_TIME(2378);
        -> '00:39:38'
mysql> select SEC_TO_TIME(2378) + 0;
        -> 3938
TIME_TO_SEC(time)
Returns the time argument, converted to seconds.
mysql> select TIME_TO_SEC('22:23:00');
        -> 80580
mysql> select TIME_TO_SEC('00:39:38');
        -> 2378

7.3.12 Miscellaneous functions

DATABASE()
Returns the current database name.
mysql> select DATABASE();
        -> 'test'
If there is no current database, DATABASE() returns the empty string.
USER()
SYSTEM_USER()
SESSION_USER()
Returns the current MySQL user name.
mysql> select USER();
        -> 'davida@localhost'
In MySQL 3.22.11 or later, this includes the client hostname as well as the username. You can extract just the username part like this (which works whether or not the value includes a hostname part):
mysql> select substring_index(USER(),"@",1);
        -> 'davida'
PASSWORD(str)
Calculates a password string from the plaintext password str. This is the function that is used for encrypting MySQL passwords for storage in the Password column of the user grant table.
mysql> select PASSWORD('badpwd');
        -> '7f84554057dd964b'
PASSWORD() encryption is non-reversible. PASSWORD() does not perform password encryption in the same way that Unix passwords are encrypted. You should not assume that if your Unix password and your MySQL password are the same, PASSWORD() will result in the same encrypted value as is stored in the Unix password file. See ENCRYPT().
ENCRYPT(str[,salt])
Encrypt str using the Unix crypt() system call. The salt argument should be a string with 2 characters.
mysql> select ENCRYPT("hello");
        -> 'VxuFAJXVARROc'
If crypt() is not available on your system, ENCRYPT() always returns NULL. ENCRYPT() ignores all but the first 8 characters of str, at least on some systems. This will be determined by the behavior of the underlying crypt() system call.
ENCODE(str,pass_str)
Encrypt str using pass_str as the password. To decrypt the result, use DECODE(). The results is a binary string. If you want to save it in a column, use a BLOB column type.
DECODE(crypt_str,pass_str)
Descrypts the encrypted string crypt_str using pass_str as the password. crypt_str should be a string returned from ENCODE().
MD5(string)
Calculates a MD5 checksum for the string. Value is returned as a 32 long hex number that may, for example, be used as a hash key.
mysql> select MD5("testing")
        -> 'ae2b1fca515949e5d54fb22b8ed95575'
This is a "RSA Data Security, Inc. MD5 Message-Digest Algorithm".
LAST_INSERT_ID([expr])
Returns the last automatically generated value that was inserted into an AUTO_INCREMENT column. See section 20.4.29 mysql_insert_id().
mysql> select LAST_INSERT_ID();
        -> 195
The last ID that was generated is maintained in the server on a per-connection basis. It will not be changed by another client. It will not even be changed if you update another AUTO_INCREMENT column with a non-magic value (that is, a value that is not NULL and not 0). If expr is given as an argument to LAST_INSERT_ID() in an UPDATE clause, then the value of the argument is returned as a LAST_INSERT_ID() value. This can be used to simulate sequences: First create the table:
mysql> create table sequence (id int not null);
mysql> insert into sequence values (0);
Then the table can be used to generate sequence numbers like this:
mysql> update sequence set id=LAST_INSERT_ID(id+1);
You can generate sequences without calling LAST_INSERT_ID(), but the utility of using the function this way is that the ID value is maintained in the server as the last automatically generated value. You can retrieve the new ID as you would read any normal AUTO_INCREMENT value in MySQL. For example, LAST_INSERT_ID() (without an argument) will return the new ID. The C API function mysql_insert_id() can also be used to get the value.
FORMAT(X,D)
Formats the number X to a format like '#,###,###.##' with D decimals. If D is 0, the result will have no decimal point or fractional part.
mysql> select FORMAT(12332.1234, 2);
        -> '12,332.12'
mysql> select FORMAT(12332.1,4);
        -> '12,332.1000'
mysql> select FORMAT(12332.2,0);
        -> '12,332'
VERSION()
Returns a string indicating the MySQL server version.
mysql> select VERSION();
        -> '3.22.19b-log'
GET_LOCK(str,timeout)
Tries to obtain a lock with a name given by the string str, with a timeout of timeout seconds. Returns 1 if the lock was obtained successfully, 0 if the attempt timed out, or NULL if an error occurred (such as running out of memory or the thread was killed with mysqladmin kill). A lock is released when you execute RELEASE_LOCK(), execute a new GET_LOCK() or the thread terminates. This function can be used to implement application locks or to simulate record locks.
mysql> select GET_LOCK("lock1",10);
        -> 1
mysql> select GET_LOCK("lock2",10);
        -> 1
mysql> select RELEASE_LOCK("lock2");
        -> 1
mysql> select RELEASE_LOCK("lock1");
        -> NULL
Note that the second RELEASE_LOCK() call returns NULL because the lock "lock1" was automatically released by the second GET_LOCK() call.
RELEASE_LOCK(str)
Releases the lock named by the string str that was obtained with GET_LOCK(). Returns 1 if the lock was released, 0 if the lock wasn't locked by this thread (in which case the lock is not released) and NULL if the named lock didn't exist. The lock will not exist if it was never obtained by a call to GET_LOCK() or if it already has been released.
BENCHMARK(count,expr)
The BENCHMARK() function executes the expression expr repeatedly count times. It may be used to time how fast MySQL processes the expression. The result value is always 0. The intended use is in the mysql client, which reports query execution times.
mysql> select BENCHMARK(1000000,encode("hello","goodbye"));
+----------------------------------------------+
| BENCHMARK(1000000,encode("hello","goodbye")) |
+----------------------------------------------+
|                                            0 |
+----------------------------------------------+
1 row in set (4.74 sec)
The time reported is elapsed time on the client end, not CPU time on the server end. It may be advisable to execute BENCHMARK() several times, and interpret the result with regard to how heavily loaded the server machine is.

7.3.13 Functions for use with GROUP BY clauses

If you use a group function in a statement containing no GROUP BY clause, it is equivalent to grouping on all rows.

COUNT(expr)
Returns a count of the number of non-NULL rows retrieved by a SELECT statement.
mysql> select student.student_name,COUNT(*)
           from student,course
           where student.student_id=course.student_id
           GROUP BY student_name;

COUNT(*) is optimized to return very quickly if the SELECT retrieves from one table, no other columns are retrieved and there is no WHERE clause. For example:
mysql> select COUNT(*) from student;
COUNT(DISTINCT expr,[expr...])
Returns a count of the number of different values.
mysql> select COUNT(DISTINCT results) from student;
In MySQL you can get the number of distinct expressions combinations by giving a list of expressions. In ANSI SQL you would have to do a concatenation of all expressions inside CODE(DISTINCT ..).
AVG(expr)
Returns the average value of expr.
mysql> select student_name, AVG(test_score)
           from student
           GROUP BY student_name;
MIN(expr)
MAX(expr)
Returns the minimum or maximum value of expr. MIN() and MAX() may take a string argument; in such cases they return the minimum or maximum string value.
mysql> select student_name, MIN(test_score), MAX(test_score)
           from student
           GROUP BY student_name;
SUM(expr)
Returns the sum of expr. Note that if the return set has no rows, it returns NULL!
STD(expr)
STDDEV(expr)
Returns the standard deviation of expr. This is an extension to ANSI SQL. The STDDEV() form of this function is provided for Oracle compatability.
BIT_OR(expr)
Returns the bitwise OR of all bits in expr. The calculation is performed with 64-bit (BIGINT precision.
BIT_AND(expr)
Returns the bitwise AND of all bits in expr. The calculation is performed with 64-bit (BIGINT precision.

MySQL has extended the use of GROUP BY. You can use columns or calculations in the SELECT expressions which don't appear in the GROUP BY part. This stands for any possible value for this group. You can use this to get better performance by avoiding sorting and grouping on unnecessary items. For example, you don't need to group on customer.name in the following query:

mysql> select order.custid,customer.name,max(payments)
       from order,customer
       where order.custid = customer.custid
       GROUP BY order.custid;

In ANSI SQL, you would have to add customer.name to the GROUP BY clause. In MySQL, the name is redundant.

Don't use this feature if the columns you omit from the GROUP BY part aren't unique in the group!

In some cases, you can use MIN() and MAX() to obtain a specific column value even if it isn't unique. The following gives the value of column from the row containing the smallest value in the sort column:

substr(MIN(concat(sort,space(6-length(sort)),column),7,length(column)))

Note that if you are using MySQL 3.22 (or earlier) or if you are trying to follow ANSI SQL, you can't use expressions in GROUP BY or ORDER BY clauses. You can work around this limitation by using an alias for the expression:

mysql> SELECT id,FLOOR(value/100) AS val FROM tbl_name
           GROUP BY id,val ORDER BY val;

In MySQL 3.23 you can do:

mysql> SELECT id,FLOOR(value/100) FROM tbl_name ORDER BY RAND();

7.4 CREATE DATABASE syntax

CREATE DATABASE db_name

CREATE DATABASE creates a database with the given name. Rules for allowable database names are given in section 7.1.5 Database, table, index, column and alias names. An error occurs if the database already exists.

Databases in MySQL are implemented as directories containing files that correspond to tables in the database. Since there are no tables in a database when it is initially created, the CREATE DATABASE statement only creates a directory under the MySQL data directory.

You can also create databases with mysqladmin. See section 12.1 Overview of the different MySQL programs.

7.5 DROP DATABASE syntax

DROP DATABASE [IF EXISTS] db_name

DROP DATABASE drops all tables in the database and deletes the database. Be VERY careful with this command!

DROP DATABASE returns the number of files that were removed from the database directory. Normally, this is three times the number of tables, since each table corresponds to a `.ISD' file, a `.ISM' file and a `.frm' file.

In MySQL 3.22 or later, you can use the keywords IF EXISTS to prevent an error from occurring if the database doesn't exist.

You can also drop databases with mysqladmin. See section 12.1 Overview of the different MySQL programs.

7.6 CREATE TABLE syntax

CREATE [TEMPORARY] TABLE [IF NOT EXISTS] tbl_name (create_definition,...)
[table_options] [select_statement]

create_definition:
  col_name type [NOT NULL | NULL] [DEFAULT default_value] [AUTO_INCREMENT]
            [PRIMARY KEY] [reference_definition]
  or    PRIMARY KEY (index_col_name,...)
  or    KEY [index_name] KEY(index_col_name,...)
  or    INDEX [index_name] (index_col_name,...)
  or    UNIQUE [INDEX] [index_name] (index_col_name,...)
  or    [CONSTRAINT symbol] FOREIGN KEY index_name (index_col_name,...)
            [reference_definition]
  or    CHECK (expr)

type:
        TINYINT[(length)] [UNSIGNED] [ZEROFILL]
  or    SMALLINT[(length)] [UNSIGNED] [ZEROFILL]
  or    MEDIUMINT[(length)] [UNSIGNED] [ZEROFILL]
  or    INT[(length)] [UNSIGNED] [ZEROFILL]
  or    INTEGER[(length)] [UNSIGNED] [ZEROFILL]
  or    BIGINT[(length)] [UNSIGNED] [ZEROFILL]
  or    REAL[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    DOUBLE[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    FLOAT[(length,decimals)] [UNSIGNED] [ZEROFILL]
  or    DECIMAL(length,decimals) [UNSIGNED] [ZEROFILL]
  or    NUMERIC(length,decimals) [UNSIGNED] [ZEROFILL]
  or    CHAR(length) [BINARY]
  or    VARCHAR(length) [BINARY]
  or    DATE
  or    TIME
  or    TIMESTAMP
  or    DATETIME
  or    TINYBLOB
  or    BLOB
  or    MEDIUMBLOB
  or    LONGBLOB
  or    TINYTEXT
  or    TEXT
  or    MEDIUMTEXT
  or    LONGTEXT
  or    ENUM(value1,value2,value3,...)
  or    SET(value1,value2,value3,...)

index_col_name:
        col_name [(length)]

reference_definition:
        REFERENCES tbl_name [(index_col_name,...)]
                   [MATCH FULL | MATCH PARTIAL]
                   [ON DELETE reference_option]
                   [ON UPDATE reference_option]

reference_option:
        RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

table_options:
	type = [ISAM | MYISAM | HEAP]
or	AUTO_INCREMENT = #
or	AVG_ROW_LENGTH = #
or	CHECKSUM = [0 | 1]
or	COMMENT = "string"
or	MAX_ROWS = #
or	MIN_ROWS = #
or	PACK_KEYS = [0 | 1]
or	PASSWORD = "string"
or	DELAY_KEY_WRITE = [0 | 1]

select_statement:
	[IGNORE | REPLACE] SELECT ...  (Some legal select statement)

CREATE TABLE creates a table with the given name in the current database. Rules for allowable table names are given in section 7.1.5 Database, table, index, column and alias names. An error occurs if there is no current database or if the table already exists.

In MySQL 3.22 or later, the table name can be specified as db_name.tbl_name. This works whether or not there is a current database.

In MySQL 3.23, you can use the TEMPORARY keyword when you create a table. A temporary table will automatically be deleted if a connection dies and the name is per connection. This means that two different connections can both use the same temporary table name without conflicting with each other or with an existing table of the same name. (The existing table is hidden until the temporary table is deleted).

In MySQL 3.23 or later, you can use the keywords IF NOT EXISTS so that an error does not occur if the table already exists. Note that there is no verification that the table structures are identical.

Each table tbl_name is represented by some files in the database directory. In the case of ISAM-type tables you will get:

File Purpose
tbl_name.frm Table definition (form) file
tbl_name.ISD Data file
tbl_name.ISM Index file

For more information on the properties of the various column types, see section 7.2 Column types.

7.6.1 Silent column specification changes

In some cases, MySQL silently changes a column specification from that given in a CREATE TABLE statement. (This may also occur with ALTER TABLE.)

If you want to see whether or not MySQL used a column type other than the one you specified, issue a DESCRIBE tbl_name statement after creating or altering your table.

Certain other column type changes may occur if you compress a table using pack_isam. See section 10.17 What are the different row formats? Or, when should VARCHAR/CHAR be used?.

7.7 ALTER TABLE syntax

ALTER [IGNORE] TABLE tbl_name alter_spec [, alter_spec ...]

alter_specification:
        ADD [COLUMN] create_definition [FIRST | AFTER column_name ]
  or    ADD INDEX [index_name] (index_col_name,...)
  or    ADD PRIMARY KEY (index_col_name,...)
  or    ADD UNIQUE [index_name] (index_col_name,...)
  or    ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  or    CHANGE [COLUMN] old_col_name create_definition
  or    MODIFY [COLUMN] create_definition
  or    DROP [COLUMN] col_name
  or    DROP PRIMARY KEY
  or    DROP INDEX key_name
  or    RENAME [AS] new_tbl_name
  or    table_option

ALTER TABLE allows you to change the structure of an existing table. For example, you can add or delete columns, create or destroy indexes, change the type of existing columns, or rename columns or the table itself. You can also change the comment for the table and type of the table. See section 7.6 CREATE TABLE syntax.

If you use ALTER TABLE to change a column specification but DESCRIBE tbl_name indicates that your column was not changed, it is possible that MySQL ignored your modification for one of the reasons described in section 7.6.1 Silent column specification changes. For example, if you try to change a VARCHAR column to CHAR, MySQL will still use VARCHAR if the table contains other variable-length columns.

ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, then the original table is deleted and the new one is renamed. This is done in such a way that all updates are automatically redirected to the new table without any failed updates. While ALTER TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready.

Here is an example that shows some of the uses of ALTER TABLE. We begin with a table t1 that is created as shown below:

mysql> CREATE TABLE t1 (a INTEGER,b CHAR(10));

To rename the table from t1 to t2:

mysql> ALTER TABLE t1 RENAME t2;

To change column a from INTEGER to TINYINT NOT NULL (leaving the name the same), and to change column b from CHAR(10) to CHAR(20) as well as renaming it from b to c:

mysql> ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);

To add a new TIMESTAMP column named d:

mysql> ALTER TABLE t2 ADD d TIMESTAMP;

To add an index on column d, and make column a the primary key:

mysql> ALTER TABLE t2 ADD INDEX (d), ADD PRIMARY KEY (a);

To remove column c:

mysql> ALTER TABLE t2 DROP COLUMN c;

To add a new AUTO_INCREMENT integer column named c:

mysql> ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT,
           ADD INDEX (c);

Note that we indexed c, because AUTO_INCREMENT columns must be indexed, and also that we declare c as NOT NULL, because indexed columns cannot be NULL.

When you add an AUTO_INCREMENT column, column values are filled in with sequence numbers for you automatically.

7.8 OPTIMIZE TABLE syntax

OPTIMIZE TABLE tbl_name

OPTIMZE TABLE should be used if you have deleted a large part of a table or if you have made many changes to a table with variable-length rows (tables that have VARCHAR, BLOB or TEXT columns). Deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. You can use OPTIMIZE TABLE to reclaim the unused space.

OPTIMIZE TABLE works by making a temporary copy of the original table. The old table is copied to the new table (without the unused rows), then the original table is deleted and the new one is renamed. This is done in such a way that all updates are automatically redirected to the new table without any failed updates. While OPTIMIZE TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready.

7.9 DROP TABLE syntax

DROP TABLE [IF EXISTS] tbl_name [, tbl_name,...]

DROP TABLE removes one or more tables. All table data and the table definition are removed, so be careful with this command!

In MySQL 3.22 or later, you can use the keywords IF EXISTS to prevent an error from occurring for tables that don't exist.

7.10 DELETE syntax

DELETE [LOW_PRIORITY] FROM tbl_name
    [WHERE where_definition] [LIMIT rows]

DELETE deletes rows from tbl_name that satisfy the condition given by where_definition, and returns the number of records deleted.

If you issue a DELETE with no WHERE clause, all rows are deleted. MySQL does this by recreating the table as an empty table, which is much faster than deleting each row. In this case, DELETE returns zero as the number of affected records. (MySQL can't return the number of rows that were actually deleted, since the recreate is done without opening the data files. As long as the table definition file `tbl_name.frm' is valid, the table can be recreated this way, even if the data or index files have become corrupted.).

If you really want to know how many records are deleted when you are deleting all rows, and are willing to suffer a speed penalty, you can use a DELETE statement of this form:

mysql> DELETE FROM tbl_name WHERE 1>0;

Note that this is MUCH slower than DELETE FROM tbl_name with no WHERE clause, because it deletes rows one at a time.

If you specify the keyword LOW_PRIORITY, execution of the DELETE is delayed until no other clients are reading from the table.

Deleted records are maintained in a linked list and subsequent INSERT operations reuse old record positions. To reclaim unused space and reduce file sizes, use the OPTIMIZE TABLE statement or the isamchk utility to reorganize tables. OPTIMIZE TABLE is easier, but isamchk is faster. See section 7.8 OPTIMIZE TABLE syntax, and section 13.4.3 Table optimization.

The MySQL-specific LIMIT rows option to DELETE tells the server the maximum number of rows to be deleted before control is returned to the client. This can be used to ensure that a specific DELETE command doesn't take too much time. You can simply repeat the DELETE command until the number of affected rows is less than the LIMIT value.

7.11 SELECT syntax

SELECT [STRAIGHT_JOIN] [SQL_SMALL_RESULT] [DISTINCT | DISTINCTROW | ALL]
    select_expression,...
    [INTO OUTFILE 'file_name' export_options]
    [FROM table_references
        [WHERE where_definition]
        [GROUP BY col_name,...]
        [HAVING where_definition]
        [ORDER BY {unsigned_integer | col_name | formula} [ASC | DESC] ,...]
        [LIMIT [offset,] rows]
        [PROCEDURE procedure_name] ]

SELECT is used to retrieve rows selected from one or more tables. select_expression indicates the columns you want to retrieve. SELECT may also be used to retrieve rows computed without reference to any table. For example:

mysql> SELECT 1 + 1;
         -> 2

All keywords used must be given in exactly the order shown above. For example, a HAVING clause must come after any GROUP BY clause and before any ORDER BY clause.

7.12 JOIN syntax

MySQL supports the following JOIN syntaxes for use in SELECT statements:

table_reference, table_reference
table_reference [CROSS] JOIN table_reference
table_reference INNER JOIN table_reference
table_reference STRAIGHT_JOIN table_reference
table_reference LEFT [OUTER] JOIN table_reference ON conditional_expr
table_reference LEFT [OUTER] JOIN table_reference USING (column_list)
table_reference NATURAL LEFT [OUTER] JOIN table_reference
{ oj table_reference LEFT OUTER JOIN table_reference ON conditional_expr }

The last LEFT OUTER JOIN syntax shown above exists only for compatibility with ODBC.

Some examples:

mysql> select * from table1,table2 where table1.id=table2.id;
mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id;
mysql> select * from table1 LEFT JOIN table2 USING (id);
mysql> select * from table1 LEFT JOIN table2 ON table1.id=table2.id
           LEFT JOIN table3 ON table2.id=table3.id;

See section 10.6 How MySQL optimizes LEFT JOIN.

7.13 INSERT syntax

    INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
        [INTO] tbl_name [(col_name,...)]
        VALUES (expression,...),(...),...
or  INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
        [INTO] tbl_name [(col_name,...)]
        SELECT ...
or  INSERT [LOW_PRIORITY | DELAYED] [IGNORE]
        [INTO] tbl_name
        SET col_name=expression, col_name=expression, ...

INSERT inserts new rows into an existing table. The INSERT ... VALUES form of the statement inserts rows based on explicitly-specified values. The INSERT ... SELECT form inserts rows selected from another table or tables. The INSERT ... VALUES form with multiple value lists is supported in MySQL 3.22.5 or later. The col_name=expression syntax is supported in MySQL 3.22.10 or later.

tbl_name is the table into which rows should be inserted. The column name list or the SET clause indicates which columns the statement specifies values for.

If you use INSERT ... SELECT or a INSERT ... VALUES statement with multiple value lists, you can use the C API function mysql_info() to get information about the query. The format of the information string is shown below:

Records: 100 Duplicates: 0 Warnings: 0

Duplicates indicates the number of rows that couldn't be inserted because they would duplicate some existing unique index value. Warnings indicates the number of attempts to insert column values that were problematic in some way. Warnings can occur under any of the following conditions:

The DELAYED option for the INSERT statement is a MySQL-specific option that is very useful if you have clients that can't wait for the INSERT to complete. This is common when you use MySQL for logging and you also periodically run SELECT statements that take a long time to complete. DELAYED was introduced in MySQL 3.22.15. It is a MySQL extension to ANSI SQL92.

Another major benefit of using INSERT DELAYED is that inserts from many clients are bundled together and written in one block. This is much faster than doing many separate inserts.

Note that currently the queued rows are only stored in memory until they are inserted into the table. This means that if you kill mysqld the hard way (kill -9) or if mysqld dies unexpectedly, any queued rows that weren't written to disk are lost!

The following happens when you use the DELAYED option to INSERT or REPLACE. In this description, the "thread" is the thread that received an INSERT DELAYED command and "handler" is the thread that handles all INSERT DELAYED statements for a particular table.

7.14 REPLACE syntax

    REPLACE [LOW_PRIORITY | DELAYED]
        [INTO] tbl_name [(col_name,...)]
        VALUES (expression,...)
or  REPLACE [LOW_PRIORITY | DELAYED]
        [INTO] tbl_name [(col_name,...)]
        SELECT ...
or  REPLACE [LOW_PRIORITY | DELAYED]
        [INTO] tbl_name
        SET col_name=expression, col_name=expression,...

REPLACE works exactly like INSERT, except that if an old record in the table has the same value as a new record on a unique index, the old record is deleted before the new record is inserted. See section 7.13 INSERT syntax.

7.15 LOAD DATA INFILE syntax

LOAD DATA [LOCAL] INFILE 'file_name.txt' [REPLACE | IGNORE]
    INTO TABLE tbl_name
    [FIELDS
        [TERMINATED BY '\t']
        [OPTIONALLY] ENCLOSED BY "]
        [ESCAPED BY '\\' ]]
    [LINES TERMINATED BY '\n']
    [IGNORE number LINES]
    [(col_name,...)]

The LOAD DATA INFILE statement reads rows from a text file into a table at a very high speed. If the LOCAL keyword is specified, the file is read from the client host. If LOCAL is not specified, the file must be located on the server. (LOCAL is available in MySQL 3.22.6 or later.)

For security reasons, when reading text files located on the server, the files must either reside in the database directory or be readable by all. Also, to use LOAD DATA INFILE on server files, you must have the file privilege on the server host. See section 6.5 Privileges provided by MySQL.

Using LOCAL will be a bit slower than letting the server access the files directly, since the contents of the file must travel from the client host to the server host. On the other hand, you do not need the file privilege to load local files.

You can also load data files by using the mysqlimport utility; it operates by sending a LOAD DATA INFILE command to the server. The --local option causes mysqlimport to read data files from the client host. You can specify the --compress option to get better performance over slow networks if the client and server support the compressed protocol.

When locating files on the server host, the server uses the following rules:

Note that these rules mean a file given as `./myfile.txt' is read from the server's data directory, whereas a file given as `myfile.txt' is read from the database directory of the current database. Note also that for statements such as those below, the file is read from the database directory for db1, not db2:

mysql> USE db1;
mysql> LOAD DATA INFILE "./data.txt" INTO TABLE db2.my_table; 

The REPLACE and IGNORE keywords control handling of input records that duplicate existing records on unique key values. If you specify REPLACE, new rows replace existing rows that have the same unique key value. If you specify IGNORE, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.

If you load data from a local file using the LOCAL keyword, the server has no way to stop transmission of the file in the middle of the operation, so the default bahavior is the same as if IGNORE is specified.

LOAD DATA INFILE is the complement of SELECT ... INTO OUTFILE. See section 7.11 SELECT syntax. To write data from a database to a file, use SELECT ... INTO OUTFILE. To read the file back into the database, use LOAD DATA INFILE. The syntax of the FIELDS and LINES clauses is the same for both commands. Both clauses are optional, but FIELDS must precede LINES if both are specified.

If you specify a FIELDS clause, each of its subclauses (TERMINATED BY, [OPTIONALLY] ENCLOSED BY and ESCAPED BY) is also optional, except that you must specify at least one of them.

If you don't specify a FIELDS clause, the defaults are the same as if you had written this:

FIELDS TERMINATED BY '\t' ENCLOSED BY " ESCAPED BY '\\'

If you don't specify a LINES clause, the default is the same as if you had written this:

LINES TERMINATED BY '\n'

In other words, the defaults cause LOAD DATA INFILE to act as follows when reading input:

Conversely, the defaults cause SELECT ... INTO OUTFILE to act as follows when writing output:

Note that to write FIELDS ESCAPED BY '\\', you must specify two backslashes for the value to be read as a single backslash.

The IGNORE number LINES option can be used to ignore a header of column names at the start of the file:

mysql> LOAD DATA INFILE "/tmp/file_name" into table test IGNORE 1 LINES;

When you use SELECT ... INTO OUTFILE in tandem with LOAD DATA INFILE to write data from a database into a file and then read the file back into the database later, the field and line handling options for both commands must match. Otherwise, LOAD DATA INFILE will not interpret the contents of the file properly. Suppose you use SELECT ... INTO OUTFILE to write a file with fields delimited by commas:

mysql> SELECT * FROM table1 INTO OUTFILE 'data.txt'
           FIELDS TERMINATED BY ','
           FROM ...

To read the comma-delimited file back in, the correct statement would be:

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
           FIELDS TERMINATED BY ',';

If instead you tried to read in the file with the statement shown below, it wouldn't work because it instructs LOAD DATA INFILE to look for tabs between fields:

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE table2
           FIELDS TERMINATED BY '\t';

The likely result is that each input line would be interpreted as a single field.

LOAD DATA INFILE can be used to read files obtained from external sources, too. For example, a file in dBASE format will have fields separated by commas and enclosed in double quotes. If lines in the file are terminated by newlines, the command shown below illustrates the field and line handling options you would use to load the file:

mysql> LOAD DATA INFILE 'data.txt' INTO TABLE tbl_name
           FIELDS TERMINATED BY ',' ENCLOSED BY '"'
           LINES TERMINATED BY '\n';

Any of the field or line handling options may specify an empty string ("). If not empty, the FIELDS [OPTIONALLY] ENCLOSED BY and FIELDS ESCAPED BY values must be a single character. The FIELDS TERMINATED BY and LINES TERMINATED BY values may be more than one character. For example, to write lines that are terminated by carriage return-linefeed pairs, or to read a file containing such lines, specify a LINES TERMINATED BY '\r\n' clause.

FIELDS [OPTIONALLY] ENCLOSED BY controls quoting of fields. For output (SELECT ... INTO OUTFILE), if you omit the word OPTIONALLY, all fields are enclosed by the ENCLOSED BY character. An example of such output (using a comma as the field delimiter) is shown below:

"1","a string","100.20"
"2","a string containing a , comma","102.20"
"3","a string containing a \" quote","102.20"
"4","a string containing a \", quote and comma","102.20"

If you specify OPTIONALLY, the ENCLOSED BY character is used only to enclose CHAR and VARCHAR fields:

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a \" quote",102.20
4,"a string containing a \", quote and comma",102.20

Note that occurrences of the ENCLOSED BY character within a field value are escaped by prefixing them with the ESCAPED BY character. Also note that if you specify an empty ESCAPED BY value, it is possible to generate output that cannot be read properly by LOAD DATA INFILE. For example, the output just shown above would appear as shown below if the escape character is empty. Observe that the second field in the fourth line contains a comma following the quote, which (erroneously) appears to terminate the field:

1,"a string",100.20
2,"a string containing a , comma",102.20
3,"a string containing a " quote",102.20
4,"a string containing a ", quote and comma",102.20

For input, the ENCLOSED BY character, if present, is stripped from the ends of field values. (This is true whether or not OPTIONALLY is specified; OPTIONALLY has no effect on input interpretation.) Occurrences of the ENCLOSED BY character preceded by the ESCAPED BY character are interpreted as part of the current field value. In addition, duplicated ENCLOSED BY characters occurring within fields are interpreted as single ENCLOSED BY characters if the field itself starts with that character. For example, if ENCLOSED BY '"' is specified, quotes are handled as shown below:

"The ""BIG"" boss"  -> The "BIG" boss
The "BIG" boss      -> The "BIG" boss
The ""BIG"" boss    -> The ""BIG"" boss

FIELDS ESCAPED BY controls how to write or read special characters. If the FIELDS ESCAPED BY character is not empty, it is used to prefix the following characters on output:

If the FIELDS ESCAPED BY character is empty, no characters are escaped. It is probably not a good idea to specify an empty escape character, particularly if field values in your data contain any of the characters in the list just given.

For input, if the FIELDS ESCAPED BY character is not empty, occurrences of that character are stripped and the following character is taken literally as part of a field value. The exceptions are an escaped `0' or `N' (e.g., \0 or \N if the escape character is `\'). These sequences are interpreted as ASCII 0 (a zero-valued byte) and NULL. See below for the rules on NULL handling.

For more information about `\'-escape syntax, see section 7.1 Literals: how to write strings and numbers.

In certain cases, field and line handling options interact:

Handling of NULL values varies, depending on the FIELDS and LINES options you use:

Some cases are not supported by LOAD DATA INFILE:

The following example loads all columns of the persondata table:

mysql> LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata;

No field list is specified, so LOAD DATA INFILE expects input rows to contain a field for each table column. The default FIELDS and LINES values are used.

If you wish to load only some of a table's columns, specify a field list:

mysql> LOAD DATA INFILE 'persondata.txt'
           INTO TABLE persondata (col1,col2,...);

You must also specify a field list if the order of the fields in the input file differs from the order of the columns in the table. Otherwise, MySQL cannot tell how to match up input fields with table columns.

If a row has too few fields, the columns for which no input field is present are set to default values. Default value assignment is described in section 7.6 CREATE TABLE syntax.

An empty field value is interpreted differently than if the field value is missing:

TIMESTAMP columns are only set to the current date and time if there is a NULL value for the column, or (for the first TIMESTAMP column only) if the TIMESTAMP column is left out from the field list when a field list is specified.

If an input row has too many fields, the extra fields are ignored and the number of warnings is incremented.

LOAD DATA INFILE regards all input as strings, so you can't use numeric values for ENUM or SET columns the way you can with INSERT statements. All ENUM and SET values must be specified as strings!

If you are using the C API, you can get information about the query by calling the API function mysql_info() when the LOAD DATA INFILE query finishes. The format of the information string is shown below:

Records: 1  Deleted: 0  Skipped: 0  Warnings: 0

Warnings occur under the same circumstances as when values are inserted via the INSERT statement (see section 7.13 INSERT syntax), except that LOAD DATA INFILE also generates warnings when there are too few or too many fields in the input row. The warnings are not stored anywhere; the number of warnings can only be used as an indication if everything went well. If you get warnings and want to know exactly why you got them, one way to do this is to use SELECT ... INTO OUTFILE into another file and compare this to your original input file.

For more information about the efficiency of INSERT versus LOAD DATA INFILE and speeding up LOAD DATA INFILE, see section 10.12 How to arrange a table to be as fast/small as possible.

7.16 UPDATE syntax

UPDATE [LOW_PRIORITY] tbl_name SET col_name1=expr1,col_name2=expr2,...
    [WHERE where_definition] [LIMIT #]

UPDATE updates columns in existing table rows with new values. The SET clause indicates which columns to modify and the values they should be given. The WHERE clause, if given, specifies which rows should be updated. Otherwise all rows are updated.

If you specify the keyword LOW_PRIORITY, execution of the UPDATE is delayed until no other clients are reading from the table.

If you access a column from tbl_name in an expression, UPDATE uses the current value of the column. For example, the following statement sets the age column to one more than its current value:

mysql> UPDATE persondata SET age=age+1;

UPDATE assignments are evaluated from left to right. For example, the following statement doubles the age column, then increments it:

mysql> UPDATE persondata SET age=age*2, age=age+1;

If you set a column to the value it currently has, MySQL notices this and doesn't update it.

UPDATE returns the number of rows that were actually changed. In MySQL 3.22 or later, the C API function mysql_info() returns the number of rows that were matched and updated and the number of warnings that occurred during the UPDATE.

In MySQL 3.23 you can use LIMIT # to ensure that only a given number of rows are changed.

7.17 USE syntax

USE db_name

The USE db_name statement tells MySQL to use the db_name database as the default database for subsequent queries. The database remains current until the end of the session, or until another USE statement is issued:

mysql> USE db1;
mysql> SELECT count(*) FROM mytable;      # selects from db1.mytable
mysql> USE db2;
mysql> SELECT count(*) FROM mytable;      # selects from db2.mytable

Making a particular database current by means of the USE statement does not preclude you from accessing tables in other databases. The example below accesses the author table from the db1 database and the editor table from the db2 database:

mysql> USE db1;
mysql> SELECT author_name,editor_name FROM author,db2.editor
           WHERE author.editor_id = db2.editor.editor_id;

The USE statement is provided for Sybase compatibility.

7.18 FLUSH syntax (clearing caches)

FLUSH flush_option [,flush_option]

You should use the FLUSH command if you want to clear some of the internal caches MySQL uses. To execute FLUSH, you must have the reload privilege.

flush_option can be any of the following:

HOSTS Empties the host cache tables. You should flush the host tables if some of your hosts change IP number or if you get the error message Host ... is blocked. When more than max_connect_errors errors occur in a row for a given host while connection to the MySQL server, MySQL assumes something is wrong and blocks the host from further connection requests. Flushing the host tables allows the host to attempt to connect again. See section 18.2.3 Host '...' is blocked error.) You can start mysqld with -O max_connection_errors=999999999 to avoid this error message.
LOGS Closes and reopens the standard and update log files. If you have specified the update log file without an extension, the extension number of the new update log file will be incremented by one relative to the previous file.
PRIVILEGES Reloads the privileges from the grant tables in the mysql database.
TABLES Closes all open tables.
STATUS Resets most status variables to zero.

You can also access each of the commands shown above with the mysqladmin utility, using the flush-hosts, flush-logs, reload or flush-tables commands.

7.19 KILL syntax

KILL thread_id

Each connection to mysqld runs in a separate thread. You can see which threads are running with the SHOW PROCESSLIST command, and kill a thread with the KILL thread_id command.

If you have the process privilege, you can see and kill all threads. Otherwise, you can see and kill only your own threads.

You can also use the mysqladmin processlist and mysqladmin kill commands to examine and kill threads.

7.20 SHOW syntax (Get information about tables, columns,...)

   SHOW DATABASES [LIKE wild]
or SHOW TABLES [FROM db_name] [LIKE wild]
or SHOW COLUMNS FROM tbl_name [FROM db_name] [LIKE wild]
or SHOW INDEX FROM tbl_name [FROM db_name]
or SHOW STATUS
or SHOW VARIABLES [LIKE wild]
or SHOW PROCESSLIST
or SHOW TABLE STATUS [FROM db_name] [LIKE wild]

SHOW provides information about databases, tables, columns or the server. If the LIKE wild part is used, the wild string can be a string that uses the SQL `%' and `_' wildcard characters.

You can use db_name.tbl_name as an alternative to the tbl_name FROM db_name syntax. These two statements are equivalent:

mysql> SHOW INDEX FROM mytable FROM mydb;
mysql> SHOW INDEX FROM mydb.mytable;

SHOW DATABASES lists the databases on the MySQL server host. You can also get this list using the mysqlshow command.

SHOW TABLES lists the tables in a given database. You can also get this list using the mysqlshow db_name command.

Note: If a user doesn't have any privileges for a table, the table will not show up in the output from SHOW TABLES or mysqlshow db_name.

SHOW COLUMNS lists the columns in a given table. If the column types are different than you expect them to be based on a CREATE TABLE statement, note that MySQL sometimes changes column types. See section 7.6.1 Silent column specification changes.

The DESCRIBE statement provides information similar to SHOW COLUMNS. See section 7.22 DESCRIBE syntax (Get information about columns).

SHOW TABLE STATUS (new in version 3.23) works likes SHOW STATUS, but provides a lot of information about each table. You can also get this list using the mysqlshow --status db_name command. The following columns are returned:

Name Name of the table
Type Type of table (NISAM, MyISAM or HEAP)
Rows Number of rows
Avg_row_length Average row length
Data_length Length of the data file
Max_data_length Max length of the data file
Index_length Length of the index file
Data_free Number of allocated but not used bytes
Auto_increment Next autoincrement value
Create_time When the table was created
Update_time When the data file was last updated
Check_time When one last run a check on the table
Create_options Extra options used with CREATE TABLE
Comment The comment used when creating the table (or some information why MySQL couldn't access the table information).

SHOW FIELDS is a synonym for SHOW COLUMNS and SHOW KEYS is a synonym for SHOW INDEX. You can also list a table's columns or indexes with mysqlshow db_name tbl_name or mysqlshow -k db_name tbl_name.

SHOW INDEX returns the index information in a format that closely resembles the SQLStatistics call in ODBC. The following columns are returned:

Table Name of the table
Non_unique 0 if the index can't contain duplicates.
Key_name Name of the index
Seq_in_index Column sequence number in index, starting with 1.
Column_name Column name.
Collation How the column is sorted in the index. In MySQL, this can have values A (Ascending) or NULL (Not sorted).
Cardinality Number of unique values in the index. This is updated by running isamchk -a.
Sub_part Number of indexed characters if the column is only partly indexed. NULL if the entire key is indexed.

SHOW STATUS provides server status information (like mysqladmin extended-status). The output resembles that shown below, though the format and numbers may differ somewhat:

+--------------------------+--------+
| Variable_name            | Value  |
+--------------------------+--------+
| Aborted_clients          | 0      |
| Aborted_connects         | 0      |
| Created_tmp_tables       | 0      |
| Delayed_insert_threads   | 0      |
| Delayed_writes           | 0      |
| Delayed_errors           | 0      |
| Flush_commands           | 2      |
| Handler_delete           | 2      |
| Handler_read_first       | 0      |
| Handler_read_key         | 1      |
| Handler_read_next        | 0      |
| Handler_read_rnd         | 35     |
| Handler_update           | 0      |
| Handler_write            | 2      |
| Key_blocks_used          | 0      |
| Key_read_requests        | 0      |
| Key_reads                | 0      |
| Key_write_requests       | 0      |
| Key_writes               | 0      |
| Max_used_connections     | 1      |
| Not_flushed_key_blocks   | 0      |
| Not_flushed_delayed_rows | 0      |
| Open_tables              | 1      |
| Open_files               | 2      |
| Open_streams             | 0      |
| Opened_tables            | 11     |
| Questions                | 14     |
| Running_threads          | 1      |
| Slow_queries             | 0      |
| Uptime                   | 149111 |
+--------------------------+--------+

The status variables listed above have the following meaning:

Aborted_clients Number of connections that has been aborted because the client has died without closing the connection properly.
Aborted_connects Number of tries to connect to the MySQL server that has failed.
Created_tmp_tables Number of implicit temporary tables that has been created while executing statements.
Delayed_insert_threads Number of delayed insert handler threads in use.
Delayed_writes Number of rows written with INSERT DELAYED.
Delayed_errors Number of rows written with INSERT DELAYED for which some error occurred (probably duplicate key).
Flush_commands Number of executed FLUSH commands.
Handler_delete Number of requests to delete a row from a table.
Handler_read_first Number of request to read first the row in a table.
Handler_read_key Number of request to read a row based on a key.
Handler_read_next Number of request to read next row in key order.
Handler_read_rnd Number of request to read a row based on a fixed position.
Handler_update Number of requests to update a row in a table.
Handler_write Number of requests to insert a row in a table.
Key_blocks_used The number of used blocks in the key cache.
Key_read_requests The number of request to read a key block from the cache.
Key_reads The number of physical reads of a key block from disk.
Key_write_requests The number of request to write a key block to the cache.
Key_writes The number of physical writes of a key block to disk.
Max_used_connections The maximum number of connections that has been in use simultaneously.
Not_flushed_key_blocks Keys blocks in the key cache that has changed but hasn't yet been flushed to disk.
Not_flushed_delayed_rows Number of rows waiting to be written in INSERT DELAY queues.
Open_tables Number of tables that are open.
Open_files Number of files that are open.
Open_streams Number of streams that are open (used mainly for logging)
Opened_tables Number of tables that has been opened.
Questions Number of questions asked from to the server.
Running_threads Number of currently open connections.
Slow_queries Number of queries that has taken more than long_query_time
Uptime How many seconds the server has been up.

Some comments about the above:

SHOW VARIABLES shows the values of the some of MySQL system variables. You can also get this information using the mysqladmin variables command. If the default values are unsuitable, you can set most of these variables using command-line options when mysqld starts up. The output resembles that shown below, though the format and numbers may differ somewhat:

+------------------------+--------------------------+
| Variable_name          | Value                    |
+------------------------+--------------------------+
| back_log               | 5                        |
| connect_timeout        | 5                        |
| basedir                | /my/monty/               |
| datadir                | /my/monty/data/          |
| delayed_insert_limit   | 100                      |
| delayed_insert_timeout | 300                      |
| delayed_queue_size     | 1000                     |
| join_buffer_size       | 131072                   |
| flush_time             | 0                        |
| key_buffer_size        | 1048540                  |
| language               | /my/monty/share/english/ |
| log                    | OFF                      |
| log_update             | OFF                      |
| long_query_time        | 10                       |
| low_priority_updates   | OFF                      |
| max_allowed_packet     | 1048576                  |
| max_connections        | 100                      |
| max_connect_errors     | 10                       |
| max_delayed_threads    | 20                       |
| max_heap_table_size    | 16777216                 |
| max_join_size          | 4294967295               |
| max_sort_length        | 1024                     |
| max_tmp_tables         | 32                       |
| net_buffer_length      | 16384                    |
| port                   | 3306                     |
| protocol-version       | 10                       |
| record_buffer          | 131072                   |
| skip_locking           | ON                       |
| socket                 | /tmp/mysql.sock          |
| sort_buffer            | 2097116                  |
| table_cache            | 64                       |
| thread_stack           | 131072                   |
| tmp_table_size         | 1048576                  |
| tmpdir                 | /machine/tmp/            |
| version                | 3.23.0-alpha-debug       |
| wait_timeout           | 28800                    |
+------------------------+--------------------------+

See section 10.1 Tuning server parameters.

SHOW PROCESSLIST shows you which threads are running. You can also get this information using the mysqladmin processlist command. If you have the process privilege, you can see all threads. Otherwise, you can see only your own threads. See section 7.19 KILL syntax.

7.21 EXPLAIN syntax (Get information about a SELECT)

EXPLAIN SELECT select_options

When you precede a SELECT statement with the keyword EXPLAIN, MySQL explains how it would process the SELECT, providing information about how tables are joined and in which order.

With the help of EXPLAIN, you can see when you must add indexes to tables to get a faster SELECT that uses indexes to find the records. You can also see if the optimizer joins the tables in an optimal order. To force the optimizer to use a specific join order for a SELECT statement, add a STRAIGHT_JOIN clause.

For non-simple joins, EXPLAIN returns a row of information for each table used in the SELECT statement. The tables are listed in the order they would be read. MySQL resolves all joins using a single-sweep multi-join method. This means that MySQL reads a row from the first table, then finds a matching row in the second table, then in the third table and so on. When all tables are processed, it outputs the selected columns and backtracks through the table list until a table is found for which there are more matching rows. The next row is read from this table and the process continues with the next table.

Output from EXPLAIN includes the following columns:

table
The table to which the row of output refers.
type
The join type. Information about the various types is given below.
possible_keys
The possible_keys column indicates which indexes MySQL could use to find the rows in the table. If this column is empty, there are no relevant indexes. In this case, you may be able to improve the performance of your query by examining the WHERE clause to see if it refers to some column or columns that would be suitable for indexing. If so, create an appropriate index and check the query with EXPLAIN again. To see what indexes a table has, use SHOW INDEX FROM tbl_name.
key
The key column indicates the key that MySQL actually decided to use. The key is NULL if no index was chosen.
key_len
The key_len column indicates the length of the key that MySQL decided to use. The length is NULL if the key is NULL.
ref
The ref column shows which columns or constants are used with the key to select rows from the table.
rows
The rows column indicates the number of rows MySQL must examine to execute the query.
Extra
If the Extra column includes the text Only index, this means that information is retrieved from the table using only information in the index tree. Normally, this is much faster than scanning the entire table. If the Extra column includes the text where used, it means that a WHERE clause will be used to restrict which rows will be matched against the next table or sent to the client.

The different join types are listed below, ordered from best to worst type:

system
The table has only one row (= system table). This is a special case of the const join type.
const
The table has at most one matching row, which will be read at the start of the query. Since there is only one row, values from the column in this row can be regarded as constants by the rest of the optimizer. const tables are very fast as they are read only once!
eq_ref
One row will be read from this table for each combination of rows from the previous tables. This the best possible join type, other than the const types. It is used when all parts of an index are used by the join and the index is UNIQUE or a PRIMARY KEY.
ref
All rows with matching index values will be read from this table for each combination of rows from the previous tables. ref is used if the join uses only a leftmost prefix of the key, or if the key is not UNIQUE or a PRIMARY KEY (in other words, if the join cannot select a single row based on the key value). If the key that is used matches only a few rows, this join type is good.
range
Only rows that are in a given range will be retrieved, using an index to select the rows. The ref column indicates which index is used.
index
This is the same as ALL, except that only the index tree is scanned. This is usually faster than ALL, as the index file is usually smaller than the data file.
ALL
A full table scan will be done for each combination of rows from the previous tables. This is normally not good if the table is the first table not marked const, and usually very bad in all other cases. You normally can avoid ALL by adding more indexes, so that the row can be retrieved based on constant values or column values from earlier tables.

You can get a good indication of how good a join is by multiplying all values in the rows column of the EXPLAIN output. This should tell you roughly how many rows MySQL must examine to execute the query. This number is also used when you restrict queries with the max_join_size variable. See section 10.1 Tuning server parameters.

The following example shows how a JOIN can be optimized progressively using the information provided by EXPLAIN.

Suppose you have the SELECT statement shown below, that you examine using EXPLAIN:

EXPLAIN SELECT tt.TicketNumber, tt.TimeIn,
            tt.ProjectReference, tt.EstimatedShipDate,
            tt.ActualShipDate, tt.ClientID,
            tt.ServiceCodes, tt.RepetitiveID,
            tt.CurrentProcess, tt.CurrentDPPerson,
            tt.RecordVolume, tt.DPPrinted, et.COUNTRY,
            et_1.COUNTRY, do.CUSTNAME
        FROM tt, et, et AS et_1, do
        WHERE tt.SubmitTime IS NULL
            AND tt.ActualPC = et.EMPLOYID
            AND tt.AssignedPC = et_1.EMPLOYID
            AND tt.ClientID = do.CUSTNMBR;

For this example, assume that:

Initially, before any optimizations have been performed, the EXPLAIN statement produces the following information:

table type possible_keys                key  key_len ref  rows  Extra
et    ALL  PRIMARY                      NULL NULL    NULL 74
do    ALL  PRIMARY                      NULL NULL    NULL 2135
et_1  ALL  PRIMARY                      NULL NULL    NULL 74
tt    ALL  AssignedPC,ClientID,ActualPC NULL NULL    NULL 3872
      range checked for each record (key map: 35)

Since type is ALL for each table, this output indicates that MySQL is doing a full join for all tables! This will take quite a long time, as the product of the number of rows in each table must be examined! For the case at hand, this is 74 * 2135 * 74 * 3872 = 45,268,558,720 rows. If the tables were bigger, you can only imagine how long it would take...

One problem here is that MySQL can't (yet) use indexes on columns efficiently if they are declared differently. In this context, VARCHAR and CHAR are the same unless they are declared as different lengths. Since tt.ActualPC is declared as CHAR(10) and et.EMPLOYID is declared as CHAR(15), there is a length mismatch.

To fix this disparity between column lengths, use ALTER TABLE to lengthen ActualPC from 10 characters to 15 characters:

mysql> ALTER TABLE tt MODIFY ActualPC VARCHAR(15);

Now tt.ActualPC and et.EMPLOYID are both VARCHAR(15). Executing the EXPLAIN statement again produces this result:

table type   possible_keys   key     key_len ref         rows    Extra
tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL 3872    where used
do    ALL    PRIMARY         NULL    NULL    NULL        2135
      range checked for each record (key map: 1)
et_1  ALL    PRIMARY         NULL    NULL    NULL        74
      range checked for each record (key map: 1)
et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC 1

This is not perfect, but is much better (the product of the rows values is now less by a factor of 74). This version is executed in a couple of seconds.

A second alteration can be made to eliminate the column length mismatches for the tt.AssignedPC = et_1.EMPLOYID and tt.ClientID = do.CUSTNMBR comparisons:

mysql> ALTER TABLE tt MODIFY AssignedPC VARCHAR(15),
                      MODIFY ClientID   VARCHAR(15);

Now EXPLAIN produces the output shown below:

table type   possible_keys   key     key_len ref            rows     Extra
et    ALL    PRIMARY         NULL    NULL    NULL           74
tt    ref    AssignedPC,ClientID,ActualPC ActualPC 15 et.EMPLOYID 52 where used
et_1  eq_ref PRIMARY         PRIMARY 15      tt.AssignedPC  1
do    eq_ref PRIMARY         PRIMARY 15      tt.ClientID    1

This is "almost" as good as it can get.

The remaining problem is that, by default, MySQL assumes that values in the tt.ActualPC column are evenly distributed, and that isn't the case for the tt table. Fortunately, it is easy to tell MySQL about this:

shell> isamchk --analyze PATH_TO_MYSQL_DATABASE/tt
shell> mysqladmin refresh

Now the join is "perfect", and EXPLAIN produces this result:

table type   possible_keys   key     key_len ref            rows    Extra
tt    ALL    AssignedPC,ClientID,ActualPC NULL NULL NULL    3872    where used
et    eq_ref PRIMARY         PRIMARY 15      tt.ActualPC    1
et_1  eq_ref PRIMARY         PRIMARY 15      tt.AssignedPC  1
do    eq_ref PRIMARY         PRIMARY 15      tt.ClientID    1

Note that the rows column in the output from EXPLAIN is an "educated guess" from the MySQL join optimizer; To optimize a query, you should check if the numbers are even close to the truth. If not, you may get better performance by using STRAIGHT_JOIN in your SELECT statement and trying to list the tables in a different order in the FROM clause.

7.22 DESCRIBE syntax (Get information about columns)

{DESCRIBE | DESC} tbl_name {col_name | wild}

DESCRIBE provides information about a table's columns. col_name may be a column name or a string containing the SQL `%' and `_' wildcard characters.

If the column types are different than you expect them to be based on a CREATE TABLE statement, note that MySQL sometimes changes column types. See section 7.6.1 Silent column specification changes.

This statement is provided for Oracle compatibility.

The SHOW statement provides similar information. See section 7.20 SHOW syntax (Get information about tables, columns,...).

7.23 LOCK TABLES/UNLOCK TABLES syntax

LOCK TABLES tbl_name [AS alias] {READ | [LOW_PRIORITY] WRITE}
            [, tbl_name {READ | [LOW_PRIORITY] WRITE} ...]
...
UNLOCK TABLES

LOCK TABLES locks tables for the current thread. UNLOCK TABLES releases any locks held by the current thread. All tables that are locked by the current thread are automatically unlocked when the thread issues another LOCK TABLES, or when the connection to the server is closed.

If a thread obtains a READ lock on a table, that thread (and all other threads) can only read from the table. If a thread obtains a WRITE lock on a table, then only the thread holding the lock can READ from or WRITE to the table. Other threads are blocked.

Each thread waits (without timing out) until it obtains all the locks it has requested.

WRITE locks normally have higher priority than READ locks, to ensure that updates are processed as soon as possible. This means that if one thread obtains a READ lock and then another thread requests a WRITE lock, subsequent READ lock requests will wait until the WRITE thread has gotten the lock and released it. You can use LOW_PRIORITY WRITE locks to allow other threads to obtain READ locks while the thread is waiting for the WRITE lock. You should only use LOW_PRIORITY WRITE locks if you are sure that there will eventually be a time when no threads will have a READ lock.

When you use LOCK TABLES, you must lock all tables that you are going to use! If you are using a table multiple times in a query (with aliases), you must get a lock for each alias! This policy ensures that table locking is deadlock free.

Note that you should NOT lock any tables that you are using with INSERT DELAYED. This is because that in this case the INSERT is done by a separate thread.

Normally, you don't have to lock tables, as all single UPDATE statements are atomic; no other thread can interfere with any other currently executing SQL statement. There are a few cases when you would like to lock tables anyway:

By using incremental updates (UPDATE customer SET value=value+new_value) or the LAST_INSERT_ID() function, you can avoid using LOCK TABLES in many cases.

You can also solve some cases by using the user-level lock functions GET_LOCK() and RELEASE_LOCK(). These locks are saved in a hash table in the server and implemented with pthread_mutex_lock() and pthread_mutex_unlock() for high speed. See section 7.3.12 Miscellaneous functions.

See section 10.11 How MySQL locks tables, for more information on locking policy.

7.24 SET OPTION syntax

SET [OPTION] SQL_VALUE_OPTION= value, ...

SET OPTION sets various options that affect the operation of the server or your client. Any option you set remains in effect until the current session ends, or until you set the option to a different value.

CHARACTER SET character_set_name | DEFAULT
This maps all strings from and to the client with the given mapping. Currently the only option for character_set_name is cp1251_koi8, but you can easily add new mappings by editing the `sql/convert.cc' file in the MySQL source distribution. The default mapping can be restored by using a character_set_name value of DEFAULT. Note that the syntax for setting the CHARACTER SET option differs from the syntax for setting the other options.
PASSWORD = PASSWORD('some password')
Set the password for the current user. Any non-anonymous user can change his own password!
PASSWORD FOR user = PASSWORD('some password')
Set the password for a specific user on the current server host. Only a user with access to the mysql database can do this. The user should be given in user@hostname format, where user and hostname are exactly as they are listed in the User and Host columns of the mysql.user table entry. For example, if you had an entry with User and Host fields of 'bob' and '%.loc.gov', you would write:
mysql> SET PASSWORD FOR bob@"%.loc.gov" = PASSWORD("newpass");
SQL_BIG_TABLES = 0 | 1
If set to 1, all temporary tables are stored on disk rather than in memory. This will be a little slower, but you will not get the error The table tbl_name is full for big SELECT operations that require a large temporary table. The default value for a new connection is 0 (i.e., use in-memory temporary tables).
SQL_BIG_SELECTS = 0 | 1
If set to 1, MySQL will abort if a SELECT is attempted that probably will take a very long time. This is useful when an inadvisable WHERE statement has been issued. A big query is defined as a SELECT that probably will have to examine more than max_join_size rows. The default value for a new connection is 0 (which will allow all SELECT statements).
SQL_LOW_PRIORITY_UPDATES = 0 | 1
If set to 1, all INSERT, UPDATE and DELETE statements wait until there is no pending SELECT on the affected table.
SQL_SELECT_LIMIT = value | DEFAULT
The maximum number of records to return from SELECT statements. If a SELECT has a LIMIT clause, the LIMIT takes precedence over the value of SQL_SELECT_LIMIT. The default value for a new connection is "unlimited". If you have changed the limit, the default value can be restored by using a SQL_SELECT_LIMIT value of DEFAULT.
SQL_LOG_OFF = 0 | 1
If set to 1, no logging will be done to the standard log for this client, if the client has the process privilege. This does not affect the update log!
SQL_LOG_UPDATE = 0 | 1
If set to 0, no logging will be done to the update log for the client, if the client has the process privilege. This does not affect the standard log!
TIMESTAMP = timestamp_value | DEFAULT
Set the time for this client. This is used to get the original timestamp if you use the update log to restore rows.
LAST_INSERT_ID = #
Set the value to be returned from LAST_INSERT_ID(). This is stored in the update log when you use LAST_INSERT_ID() in a command that updates a table.
INSERT_ID = #
Set the value to be used by the following INSERT command when inserting an AUTO_INCREMENT value. This is mainly used with the update log.

7.25 GRANT and REVOKE syntax

GRANT priv_type [(column_list)] [, priv_type [(column_list)] ...]
    ON {tbl_name | * | *.* | db_name.*}
    TO user_name [IDENTIFIED BY 'password']
        [, user_name [IDENTIFIED BY 'password'] ...]
    [WITH GRANT OPTION]

REVOKE priv_type [(column_list)] [, priv_type [(column_list)] ...]
    ON {tbl_name | * | *.* | db_name.*}
    FROM user_name [, user_name ...]

GRANT is implemented in MySQL 3.22.11 or later. For earlier MySQL versions, the GRANT statement does nothing.

The GRANT and REVOKE commands allow system administrators to grant and revoke rights to MySQL users at four privilege levels:

Global level
Global privileges apply to all databases on a given server. These privileges are stored in the mysql.user table.
Database level
Database privileges apply to all tables in a given database. These privileges are stored in the mysql.db and mysql.host tables.
Table level
Table privileges apply to all columns in a given table. These privileges are stored in the mysql.tables_priv table.
Column level
Column privileges apply to single columns in a given table. These privileges are stored in the mysql.columns_priv table.

For examples of how GRANT works, see section 6.11 Adding new user privileges to MySQL.

For the GRANT and REVOKE statements, priv_type may be specified as any of the following:

ALL PRIVILEGES      FILE                RELOAD
ALTER               INDEX               SELECT
CREATE              INSERT              SHUTDOWN
DELETE              PROCESS             UPDATE
DROP                REFERENCES          USAGE

ALL is a synonym for ALL PRIVILEGES. REFERENCES is not yet implemented. USAGE is currently a synonym for "no privileges". It can be used when you want to create a user that has no privileges.

To revoke the grant privilege from a user, use a priv_type value of GRANT OPTION:

REVOKE GRANT OPTION ON ... FROM ...;

The only priv_type values you can specify for a table are SELECT, INSERT, UPDATE, DELETE, CREATE, DROP, GRANT, INDEX and ALTER.

The only priv_type values you can specify for a column (that is, when you use a column_list clause) are SELECT, INSERT and UPDATE.

You can set global privileges by using ON *.* syntax. You can set database privileges by using ON db_name.* syntax. If you specify ON * and you have a current database, you will set the privileges for that database. (Warning: If you specify ON * and you don't have a current database, you will affect the global privileges!)

In order to accommodate granting rights to users from arbitrary hosts, MySQL supports specifying the user_name value in the form user@host. If you want to specify a user string containing special characters (such as `-'), or a host string containing special characters or wildcard characters (such as `%'), you can quote the user or host name (e.g., 'test-user'@'test-hostname').

You can specify wildcards in the hostname. For example, user@"%.loc.gov" applies to user for any host in the loc.gov domain, and user@"144.155.166.%" applies to user for any host in the 144.155.166 class C subnet.

The simple form user is a synonym for user@"%". Note: If you allow anonymous users to connect to the MySQL server (which is the default), you should also add all local users as user@localhost because otherwise the anonymous user entry for the local host in the mysql.user table will be used when the user tries to log into the MySQL server from the local machine! Anonymous users are defined by inserting entries with User=" into the mysql.user table. You can verify if this applies to you by executing this query:

mysql> SELECT Host,User FROM mysql.user WHERE User=";

For the moment, GRANT only supports host, table, database and column names up to 60 characters long. A user name can be up to 16 characters.

The privileges for a table or column are formed from the logical OR of the privileges at each of the four privilege levels. For example, if the mysql.user table specifies that a user has a global select privilege, this can't be denied by an entry at the database, table or column level.

The privileges for a column can be calculated as follows:

global privileges
OR (database privileges AND host privileges)
OR table privileges
OR column privileges

In most cases, you grant rights to a user at only one of the privilege levels, so life isn't normally as complicated as above. :) The details of the privilege-checking procedure are presented in section 6 The MySQL access privilege system.

If you grant privileges for a user/hostname combination that does not exist in the mysql.user table, an entry is added and remains there until deleted with a DELETE command. In other words, GRANT may create user table entries, but REVOKE will not remove them; you must do that explicitly using DELETE.

In MySQL 3.22.12 or later, if a new user is created or if you have global grant privileges, the user's password will be set to the password specified by the IDENTIFIED BY clause, if one is given. If the user already had a password, it is replaced by the new one.

Warning: If you create a new user but do not specify an IDENTIFIED BY clause, the user has no password. This is insecure.

Passwords can also be set with the SET PASSWORD command. See section 7.24 SET OPTION syntax.

If you grant privileges for a database, an entry in the mysql.db table is created if needed. When all privileges for the database have been removed with REVOKE, this entry is deleted.

If a user doesn't have any privileges on a table, the table is not displayed when the user requests a list of tables (e.g., with a SHOW TABLES statement).

The WITH GRANT OPTION clause gives the user the ability to give to other users any privileges the user has at the specified privilege level. You should be careful to whom you give the grant privilege, as two users with different privileges may be able to join privileges!

You cannot grant another user a privilege you don't have yourself; the grant privilege allows you to give away only those privileges you possess.

Be aware that when you grant a user the grant privilege at a particular privilege level, any privileges the user already possesses (or is given in the future!) at that level are also grantable by that user. Suppose you grant a user the insert privilege on a database. If you then grant the select privilege on the database and specify WITH GRANT OPTION, the user can give away not only the select privilege, but also insert. If you then grant the update privilege to the user on the database, the user can give away the insert, select and update.

You should not grant alter privileges to a normal user. If you do that, the user can try to subvert the privilege system by renaming tables!

Note that if you are using table or column privileges for even one user, the server examines table and column privileges for all users and this will slow down MySQL a bit.

When mysqld starts, all privileges are read into memory. Database, table and column privileges take effect at once and user-level privileges take effect the next time the user connects. Modifications to the grant tables that you perform using GRANT or REVOKE are noticed by the server immediately. If you modify the grant tables manually (using INSERT, UPDATE, etc.), you should execute a FLUSH PRIVILEGES statement or run mysqladmin flush-privileges to tell the server to reload the grant tables. See section 6.9 When privilege changes take effect.

The biggest differences between the ANSI SQL and MySQL versions of GRANT are:

7.26 CREATE INDEX syntax

CREATE [UNIQUE] INDEX index_name ON tbl_name (col_name[(length]),... )

The CREATE INDEX statement doesn't do anything in MySQL prior to version 3.22. In 3.22 or later, CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See section 7.7 ALTER TABLE syntax.

Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See section 7.6 CREATE TABLE syntax. CREATE INDEX allows you to add indexes to existing tables.

A column list of the form (col1,col2,...) creates a multiple-column index. Index values are formed by concatenating the values of the given columns.

For CHAR and VARCHAR columns, indexes can be created that use only part of a column, using col_name(length) syntax. (On BLOB and TEXT columns the length is required). The statement shown below creates an index using the first 10 characters of the name column:

mysql> CREATE INDEX part_of_name ON customer (name(10));

Since most names usually differ in the first 10 characters, this index should not be much slower than an index created from the entire name column. Also, using partial columns for indexes can make the index file much smaller, which could save a lot of disk space and might also speed up INSERT operations!

Note that you can only add a index on a column that can have NULL values or on a BLOB/TEXT column if you are useing MySQL version 3.23.2 or newer and are using the MyISAM table type.

For more information about how MySQL uses indexes, see section 10.4 How MySQL uses indexes.

7.27 DROP INDEX syntax

DROP INDEX index_name

DROP INDEX doesn't do anything in MySQL prior to version 3.22. In 3.22 or later, DROP INDEX is mapped to an ALTER TABLE statement to drop the index. See section 7.7 ALTER TABLE syntax.

7.28 Comment syntax

The MySQL server supports the # to end of line, -- to end of line and /* in-line or multiple-line */ comment styles:

mysql> select 1+1;     # This comment continues to the end of line
mysql> select 1+1;     -- This comment continues to the end of line 
mysql> select 1 /* this is an in-line comment */ + 1;
mysql> select 1+
/*
this is a
multiple-line comment
*/
1;

Note that the -- comment style requires you to have at least one space after the --!

Although the server understands the comment syntax just described, there are some limitations on the way that the mysql client parses /* ... */ comments:

These limitations apply both when you run mysql interactively and when you put commands in a file and tell mysql to read its input from that file with mysql < some-file.

MySQL doesn't support the `--' ANSI SQL comment style. See section 5.3.7 `--' as the start of a comment.

7.29 CREATE FUNCTION/DROP FUNCTION syntax

CREATE FUNCTION function_name RETURNS {STRING|REAL|INTEGER}
       SONAME shared_library_name

DROP FUNCTION function_name

A user-definable function (UDF) is a way to extend MySQL with a new function that works like native (built in) MySQL functions such as ABS() and CONCAT().

CREATE FUNCTION saves the function's name, type and shared library name in the mysql.func system table. You must have the insert and delete privileges for the mysql database to create and drop functions.

All active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable. (An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION.)

For instructions on writing user-definable functions, see section 14 Adding new functions to MySQL. For the UDF mechanism to work, functions must be written in C or C++ and your operating system must support dynamic loading.

7.30 Is MySQL picky about reserved words?

A common problem stems from trying to create a table with column names that use the names of datatypes or functions built into MySQL, such as TIMESTAMP or GROUP. You're allowed to do it (for example, ABS is an allowed column name), but whitespace is not allowed between a function name and the `(' when using functions whose names are also column names.

The following words are explicitly reserved in MySQL. Most of them are forbidden by ANSI SQL92 as column and/or table names (for example, group). A few are reserved because MySQL needs them and is (currently) using a yacc parser:

action add all alter
after and as asc
auto_increment between bigint bit
binary blob bool both
by cascade char character
change check column columns
constraint create cross current_date
current_time current_timestamp data database
databases date datetime day
day_hour day_minute day_second dayofmonth
dayofweek dayofyear dec decimal
default delete desc describe
distinct distinctrow double drop
escaped enclosed enum explain
exists fields first float
float4 float8 foreign from
for full function grant
group having hour hour_minute
hour_second ignore in index
infile insert int integer
interval int1 int2 int3
int4 int8 into if
is join key keys
last_insert_id leading left like
lines limit load lock
long longblob longtext low_priority
match mediumblob mediumtext mediumint
middleint minute minute_second month
monthname natural numeric no
not null on option
optionally or order outer
outfile partial password precision
primary procedure processlist privileges
quarter read real references
rename regexp reverse repeat
replace restrict returns rlike
second select set show
smallint soname sql_big_tables sql_big_selects
sql_select_limit sql_low_priority_updates sql_log_off sql_log_update
straight_join starting status string
table tables terminated text
time timestamp tinyblob tinytext
tinyint trailing to use
using unique unlock unsigned
update usage values varchar
variables varying varbinary with
write where year year_month
zerofill

The following symbols (from the table above) are disallowed by ANSI SQL but allowed by MySQL as column/table names. This is because some of these names are very natural names and a lot of people have already used them.

8 MySQL Tutorial

This chapter provides a tutorial introduction to MySQL by showing how to use the mysql client program to create and use a simple database. mysql (sometimes referred to as the "terminal monitor" or just "monitor") is an interactive program that allows you to connect to a MySQL server, run queries and view the results. mysql may also be used in batch mode: you place your queries in a file beforehand, then tell mysql to execute the contents of the file. Both ways of using mysql are covered here.

To see a list of options provided by mysql, invoke it with the --help option:

shell> mysql --help

This chapter assumes that mysql is installed on your machine, and that a MySQL server is available to which you can connect. If this is not true, contact your MySQL administrator. (If you are the administrator, you will need to consult other sections of this manual.)

The chapter describes the entire process of setting up and using a database. If you are interested only in accessing an already-existing database, you may want to skip over the sections that describe how to create the database and the tables it contains.

Since this chapter is tutorial in nature, many details are necessarily left out. Consult the relevant sections of the manual for more information on the topics covered here.

8.1 Connecting to and disconnecting from the server

To connect to the server, you'll usually need to provide a MySQL user name when you invoke mysql and, most likely, a password. If the server runs on a machine other than the one where you log in, you'll also need to specify a hostname. Contact your administrator to find out what connection parameters you should use to connect (i.e., what host, user name and password to use). Once you know the proper parameters, you should be able to connect like this:

shell> mysql -h host -u user -p
Enter password: ********

The ******** represents your password; enter it when mysql displays the Enter password: prompt.

If that works, you should see some introductory information followed by a mysql> prompt:

shell> mysql -h host -u user -p
Enter password: ********
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 459 to server version: 3.22.20a-log

Type 'help' for help.

mysql> 

The prompt tells you that mysql is ready for you to enter commands.

Some MySQL installations allow users to connect as the "anonymous" (unnamed) user to the server running on the local host. If this is the case on your machine, you should be able to connect to that server by invoking mysql without any options:

shell> mysql

After you have connected successfully, you can disconnect any time by typing QUIT at the mysql> prompt:

mysql> QUIT
Bye

You can also disconnect by typing control-D.

Most examples in the following sections assume you are connected to the server. They indicate this by the mysql> prompt.

8.2 Entering queries

Make sure you are connected to the server, as discussed in the previous section. Doing so will not in itself select any database to work with, but that's okay. At this point, it's more important to find out a little about how to issue queries than to jump right in creating tables, loading data into them and retrieving data from them. This section describes the basic principles of entering commands, using several queries you can try out to familiarize yourself with how mysql works.

Here's a simple command that asks the server to tell you its version number and the current date. Type it in as shown below following the mysql> prompt and hit the RETURN key:

mysql> SELECT VERSION(), CURRENT_DATE;
+--------------+--------------+
| version()    | CURRENT_DATE |
+--------------+--------------+
| 3.22.20a-log | 1999-03-19   |
+--------------+--------------+
1 row in set (0.01 sec)
mysql>

This query illustrates several things about mysql:

Keywords may be entered in any lettercase. The following queries are equivalent:

mysql> SELECT VERSION(), CURRENT_DATE;
mysql> select version(), current_date;
mysql> SeLeCt vErSiOn(), current_DATE;

Here's another query. It demonstrates that you can use mysql as a simple calculator:

mysql> SELECT SIN(PI()/4), (4+1)*5;
+-------------+---------+
| SIN(PI()/4) | (4+1)*5 |
+-------------+---------+
|    0.707107 |      25 |
+-------------+---------+

The commands shown thus far have been relatively short, single-line statements. You can even enter multiple statements on a single line. Just end each one with a semicolon:

mysql> SELECT VERSION(); SELECT NOW();
+--------------+
| version()    |
+--------------+
| 3.22.20a-log |
+--------------+

+---------------------+
| NOW()               |
+---------------------+
| 1999-03-19 00:15:33 |
+---------------------+

A command need not be given all on a single line, so lengthy commands that require several lines are not a problem. mysql determines where your statement ends by looking for the terminating semicolon, not by looking for the end of the input line. (In other words, mysql accepts free-format input: it collects input lines but does not execute them until it sees the semicolon.)

Here's a simple multiple-line statement:

mysql> SELECT
    -> USER()
    -> ,
    -> CURRENT_DATE;
+--------------------+--------------+
| USER()             | CURRENT_DATE |
+--------------------+--------------+
| joesmith@localhost | 1999-03-18   |
+--------------------+--------------+

In this example, notice how the prompt changes from mysql> to -> after you enter the first line of a multiple-line query. This is how mysql indicates that it hasn't seen a complete statement and is waiting for the rest. The prompt is your friend, because it provides valuable feedback. If you use that feedback, you will always be aware of what mysql is waiting for.

If you decide you don't want to execute a command that you are in the process of entering, cancel it by typing \c:

mysql> SELECT
    -> USER()
    -> \c
mysql>

Here, too, notice the prompt. It switches back to mysql> after you type \c, providing feedback to indicate that mysql is ready for a new command.

The following table shows each of the prompts you may see and summarizes what they mean about the state that mysql is in:

Prompt Meaning
mysql> Ready for new command
-> Waiting for next line of multiple-line command
'> Waiting for next line, collecting a string that begins with a single quote (`'')
"> Waiting for next line, collecting a string that begins with a double quote (`"')

Multiple-line statements commonly occur "by accident" when you intend to issue a command on a single line, but forget the terminating semicolon. In this case, mysql waits for more input:

mysql> SELECT USER()
    ->

If this happens to you (you think you've entered a statement but the only response is a -> prompt), most likely mysql is waiting for the semicolon. If you don't notice what the prompt is telling you, you might sit there for a while before realizing what you need to do. Enter a semicolon to complete the statement, and mysql will execute it:

mysql> SELECT USER()
    -> ;
+--------------------+
| USER()             |
+--------------------+
| joesmith@localhost |
+--------------------+

The '> and "> prompts occur during string collection. In MySQL, you can write strings surrounded by either `'' or `"' characters (for example, 'hello' or "goodbye"), and mysql lets you enter strings that span multiple lines. When you see a '> or "> prompt, it means that you've entered a line containing a string that begins with a `'' or `"' quote character, but have not yet entered the matching quote that terminates the string. That's fine if you really are entering a multiple-line string, but how likely is that? Not very. More often, the '> and "> prompts indicate that you've inadvertantly left out a quote character. For example:

mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30;
    ">

If you enter this SELECT statement, then hit RETURN and wait for the result, nothing will happen. Instead of wondering, "why does this query take so long?," notice the clue provided by the "> prompt. It tells you that mysql expects to see the rest of an unterminated string. (Do you see the error in the statement? The string "Smith is missing the second quote.)

At this point, what do you do? The simplest thing is to cancel the command. However, you cannot just type \c in this case, because mysql interprets it as part of the string that it is collecting! Instead, enter the closing quote character (so mysql knows you've finished the string), then type \c:

mysql> SELECT * FROM my_table WHERE name = "Smith AND age < 30;
    "> "\c
mysql>

The prompt changes back to mysql>, indicating that mysql is ready for a new command.

It's important to know what the '> and "> prompts signify, because if you mistakenly enter an unterminated string, any further lines you type will appear to be ignored by mysql -- including a line containing QUIT! This can be quite confusing, especially if you don't know that you need to supply the terminating quote before you can cancel the current command.

8.3 Examples of common queries

Here follows examples of how to solve some common problems with MySQL.

Some of the examples uses the table 'shop' holds the prices of each article (item number) for certain traders. Supposing that each trader has a single fixed price per article, then (item, trader) is a primary key for the records.

You can create the example table as:

CREATE TABLE shop (
 article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
 dealer  CHAR(20)                 DEFAULT "     NOT NULL,
 price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL,
 PRIMARY KEY(article, dealer));

INSERT INTO shop VALUES
(1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),(3,'C',1.69),
(3,'D',1.25),(4,'D',19.95);

Okay, so the example data is:

SELECT * FROM shop

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | A      |  3.45 |
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | B      |  1.45 |
|    0003 | C      |  1.69 |
|    0003 | D      |  1.25 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

8.3.1 The maximum value for column

"What's the highest item number?"

SELECT MAX(article) AS article FROM shop

+---------+
| article |
+---------+
|       4 |
+---------+

8.3.2 The row holding the maximum of a certain column

"Find number, dealer, and price of the most expensive article"

In ANSI-SQL this is easily done with a sub-query:

SELECT article, dealer, price
FROM   shop
WHERE  price=(SELECT MAX(price) FROM shop)

In MySQL (not having sub-selects yet) just do it in two steps:

  1. Get the maximum value from the table with a SELECT statment.
  2. Using this value compile the actual query:
    SELECT article, dealer, price
    FROM   shop
    WHERE  price=19.95
    

Another solution is to sort all rows descending by price and only get the first row using the MySQL specific LIMIT clause:

SELECT article, dealer, price
FROM   shop
ORDER BY price DESC
LIMIT 1

Note: If there are several most expensive articles (e.g. each 19.95) the LIMIT solution does only show one of them!

8.3.3 Maximum of column: per group: only the values

"What's the highest price per article?"

SELECT article, MAX(price) AS price
FROM   shop
GROUP BY article

+---------+-------+
| article | price |
+---------+-------+
|    0001 |  3.99 |
|    0002 | 10.99 |
|    0003 |  1.69 |
|    0004 | 19.95 |
+---------+-------+

8.3.4 The rows holding the group-wise maximum of a certain field

"For each article find the dealer(s) with the most expensive price."

In ANSI SQL I'd do it with a sub-query like this:

SELECT article, dealer, price
FROM   shop s1
WHERE  price=(SELECT MAX(s2.price)
              FROM shop s2
              WHERE s1.article = s2.article)

In MySQL it's best do it in several steps:

  1. Get the list of (article,maxprice). See section 8.3.4 The rows holding the group-wise maximum of a certain field.
  2. For each article get the corresponding rows which have the stored maximum price.

This can easily be done with a temporary table:

CREATE TEMPORARY TABLE tmp (
        article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
        price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL);

LOCK TABLES article read;

INSERT INTO tmp SELECT article, MAX(price) FROM shop GROUP BY article;

SELECT article, dealer, price FROM shop, tmp
WHERE shop.article=tmp.articel AND shop.price=tmp.price;

UNLOCK TABLES;

DROP TABLE tmp;

If you don't use a TEMPORARY table, you must also lock the 'tmp' table.

"Can it be done with a single query?"

Yes, but only by using a quite inefficient trick that I call the "MAX-CONCAT trick":

SELECT article,
       SUBSTRING( MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 7) AS dealer,
  0.00+LEFT(      MAX( CONCAT(LPAD(price,6,'0'),dealer) ), 6) AS price
FROM   shop
GROUP BY article;

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | C      |  1.69 |
|    0004 | D      | 19.95 |
+---------+--------+-------+

The last example can of course be made a bit more efficient by doing the splitting of the concatenated column in the client.

8.3.5 Using foreign keys

One doesn't need foreign keys to join 2 tables.

The only thing MySQL doesn't do is CHECK to make sure that the keys you use really exist in the table(s) you're referencing and it doesn't automatically delete rows from table with a foreign key definition. If you use your keys like normal, and it'll work just fine!

CREATE TABLE persons (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT, 
    name CHAR(60) NOT NULL,
    PRIMARY KEY (id)
);

CREATE TABLE shirts (
    id SMALLINT UNSIGNED NOT NULL AUTO_INCREMENT,
    style ENUM('t-shirt', 'polo', 'dress') NOT NULL,
    color ENUM('red', 'blue', 'orange', 'white', 'black') NOT NULL,
    owner SMALLINT UNSIGNED NOT NULL REFERENCES persons,
    PRIMARY KEY (id)
);

INSERT INTO persons VALUES (NULL, 'Antonio Paz');

INSERT INTO shirts VALUES
(NULL, 'polo', 'blue', LAST_INSERT_ID()),
(NULL, 'dress', 'white', LAST_INSERT_ID()),
(NULL, 't-shirt', 'blue', LAST_INSERT_ID());

INSERT INTO persons VALUES (NULL, 'Lilliana Angelovska');

INSERT INTO shirts VALUES
(NULL, 'dress', 'orange', LAST_INSERT_ID()),
(NULL, 'polo', 'red', LAST_INSERT_ID()),
(NULL, 'dress', 'blue', LAST_INSERT_ID()),
(NULL, 't-shirt', 'white', LAST_INSERT_ID());

SELECT * FROM persons;
+----+---------------------+
| id | name                |
+----+---------------------+
|  1 | Antonio Paz         |
|  2 | Lilliana Angelovska |
+----+---------------------+

SELECT * FROM shirts;
+----+---------+--------+-------+
| id | style   | color  | owner |
+----+---------+--------+-------+
|  1 | polo    | blue   |     1 |
|  2 | dress   | white  |     1 |
|  3 | t-shirt | blue   |     1 |
|  4 | dress   | orange |     2 |
|  5 | polo    | red    |     2 |
|  6 | dress   | blue   |     2 |
|  7 | t-shirt | white  |     2 |
+----+---------+--------+-------+

SELECT s.* FROM persons p, shirts s
 WHERE p.name LIKE 'Lilliana%'
   AND s.owner = p.id
   AND s.color &lt;&gt; 'white';

+----+-------+--------+-------+
| id | style | color  | owner |
+----+-------+--------+-------+
|  4 | dress | orange |     2 |
|  5 | polo  | red    |     2 |
|  6 | dress | blue   |     2 |
+----+-------+--------+-------+

8.4 Creating and using a database

Now that you know how to enter commands, it's time to access a database.

Suppose you have several pets in your home (your "menagerie") and you'd like to keep track of various types of information about them. You can do so by creating tables to hold your data and loading them with the desired information. Then you can answer different sorts of questions about your animals by retrieving data from the tables. This section shows how to do all that:

The menagerie database will be simple (deliberately), but it is not difficult to think of real-world situations in which a similar type of database might be used. For example, a database like this could be used by a farmer to keep track of livestock, or by a veterinarian to keep track of patient records.

Use the SHOW statement to find out what databases currently exist on the server:

mysql> SHOW DATABASES;
+----------+
| Database |
+----------+
| mysql    |
| test     |
| tmp      |
+----------+

The list of databases is probably different on your machine, but the mysql and test databases are likely to be among them. The mysql database is required because it describes user access privileges. The test database is often provided as a workspace for users to try things out.

If the test database exists, try to access it:

mysql> USE test
Database changed

Note that USE, like QUIT, does not require a semicolon. (You can terminate such statements with a semicolon if you like; it does no harm.) The USE statement is special in another way, too: it must be given on a single line.

You can use the test database (if you have access to it) for the examples that follow, but anything you create in that database can be removed by anyone else with access to it. For this reason, you should probably ask your MySQL administrator for permission to use a database of your own. Suppose you want to call yours menagerie. The administrator needs to execute a command like this:

mysql> GRANT ALL ON menagerie.* TO your_mysql_name;

where your_mysql_name is the MySQL user name assigned to you.

8.4.1 Creating and selecting a database

If the administrator creates your database for you when setting up your permissions, you can begin using it. Otherwise, you need to create it yourself:

mysql> CREATE DATABASE menagerie;

Under Unix, database names are case sensitive (unlike SQL keywords), so you must always refer to your database as menagerie, not as Menagerie, MENAGERIE or some other variant. This is also true for table names. (Under Windows, this restriction does not apply, although you must refer to databases and tables using the same lettercase throughout a given query.)

Creating a database does not select it for use, you must do that explicitly. To make menagerie the current database, use this command:

mysql> USE menagerie
Database changed

Your database needs to be created only once, but you must select it for use each time you begin a mysql session. You can do this by issuing a USE statement as shown above. Alternatively, you can select the database on the command line when you invoke mysql. Just specify its name after any connection parameters that you might need to provide. For example:

shell> mysql -h host -u user -p menagerie
Enter password: ********

Note that menagerie is not your password on the command just shown. If you want to supply your password on the command line after the -p option, you must do so with no intervening space (e.g., as -pmypassword, not as -p mypassword). However, putting your password on the command line is not recommended, because doing so exposes it to snooping by other users logged in on your machine.

8.4.2 Creating a table

Creating the database is the easy part, but at this point it's empty, as SHOW TABLES will tell you:

mysql> SHOW TABLES;
Empty set (0.00 sec)

The harder part is deciding what the structure of your database should be: what tables you will need, and what columns will be in each of them.

You'll want a table that contains a record for each of your pets. This can be called the pet table, and it should contain, as a bare minimum, each animal's name. Since the name by itself is not very interesting, the table should contain other information. For example, if more than one person in your family keeps pets, you might want to list each animal's owner. You might also want to record some basic descriptive information such as species and sex.

How about age? That might be of interest, but it's not a good thing to store in a database. Age changes as time passes, which means you'd have to update your records often. Instead, it's better to store a fixed value such as date of birth. Then, whenever you need age, you can calculate it as the difference between the current date and the birth date. MySQL provides functions for doing date arithmetic, so this is not difficult. Storing birth date rather than age has other advantages, too:

You can probably think of other types of information that would be useful in the pet table, but the ones identified so far are sufficient for now: name, owner, species, sex, birth and death.

Use a CREATE TABLE statement to specify the layout of your table:

mysql> CREATE TABLE pet (name VARCHAR(20), owner VARCHAR(20),
    -> species VARCHAR(20), sex CHAR(1), birth DATE, death DATE);

VARCHAR is a good choice for the name, owner and species columns since the column values will vary in length. The lengths of those columns need not all be the same, and need not be 20. You can pick any length from 1 to 255, whatever seems most reasonable to you. (If you make a poor choice and it turns out later that you need a longer field, MySQL provides an ALTER TABLE statement.)

Animal sex can be represented in a variety of ways, for example, "m" and "f", or perhaps "male" and "female". It's simplest to use the single characters "m" and "f".

The use of the DATE data type for the birth and death columns is a fairly obvious choice.

Now that you have created a table, SHOW TABLES should produce some output:

mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| pet                 |
+---------------------+

To verify that your table was created the way you expected, use a DESCRIBE statement:

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(20) | YES  |     | NULL    |       |
| owner   | varchar(20) | YES  |     | NULL    |       |
| species | varchar(20) | YES  |     | NULL    |       |
| sex     | char(1)     | YES  |     | NULL    |       |
| birth   | date        | YES  |     | NULL    |       |
| death   | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

You can use DESCRIBE any time, for example, if you forget the names of the columns in your table or what types they are.

8.4.3 Loading data into a table

After creating your table, you need to populate it. The LOAD DATA and INSERT statements are useful for this.

Suppose your pet records can be described as shown below. (Observe that MySQL expects dates in YYYY-MM-DD format; this may be different than what you are used to.)

name owner species sex birth death
Fluffy Harold cat f 1993-02-04
Claws Gwen cat m 1994-03-17
Buffy Harold dog f 1989-05-13
Fang Benny dog m 1990-08-27
Bowser Diane dog m 1998-08-31 1995-07-29
Chirpy Gwen bird f 1998-09-11
Whistler Gwen bird 1997-12-09
Slim Benny snake m 1996-04-29

Since you are beginning with an empty table, an easy way to populate it is to create a text file containing a row for each of your animals, then load the contents of the file into the table with a single statement.

You could create a text file `pet.txt' containing one record per line, with values separated by tabs, and given in the order in which the columns were listed in the CREATE TABLE statement. For missing values (such as unknown sexes, or death dates for animals that are still living), you can use NULL values. To represent these in your text file, use \N. For example, the record for Whistler the bird would look like this (where the whitespace between values is a single tab character):

Whistler Gwen bird \N 1997-12-09 \N

To load the text file `pet.txt' into the pet table, use this command:

mysql> LOAD DATA LOCAL INFILE "pet.txt" INTO TABLE pet;

You can specify the column value separator and end of line marker explicitly in the LOAD DATA statement if you wish, but the defaults are tab and linefeed. These are sufficient for the statement to read the file `pet.txt' properly.

When you want to add new records one at a time, the INSERT statement is useful. In its simplest form, you supply values for each column, in the order in which the columns were listed in the CREATE TABLE statement. Suppose Diane gets a new hamster named Puffball. You could add a new record using an INSERT statement like this:

mysql> INSERT INTO pet
    -> VALUES ('Puffball','Diane','hamster','f','1999-03-30',NULL);

Note that string and date values are specified as quoted strings here. Also, with INSERT, you can insert NULL directly to represent a missing value. You do not use \N like you do with LOAD DATA.

From this example, you should be able to see that there would be a lot more typing involved to load your records initially using several INSERT statements rather than a single LOAD DATA statement.

8.4.4 Retrieving information from a table

The SELECT statement is used to pull information from a table. The general form of the statement is:

SELECT what_to_select
FROM which_table
WHERE conditions_to_satisfy

what_to_select indicates what you want to see. This can be a list of columns, or * to to indicate "all columns." which_table indicates the table from which you want to retrieve data. The WHERE clause is optional. If it's present, conditions_to_satisfy specifies conditions that rows must satisfy to qualify for retrieval.

8.4.4.1 Selecting all data

The simplest form of SELECT retrieves everything from a table:

mysql> SELECT * FROM pet;
+----------+--------+---------+------+------------+------------+
| name     | owner  | species | sex  | birth      | death      |
+----------+--------+---------+------+------------+------------+
| Fluffy   | Harold | cat     | f    | 1993-02-04 | NULL       |
| Claws    | Gwen   | cat     | m    | 1994-03-17 | NULL       |
| Buffy    | Harold | dog     | f    | 1989-05-13 | NULL       |
| Fang     | Benny  | dog     | m    | 1990-08-27 | NULL       |
| Bowser   | Diane  | dog     | m    | 1998-08-31 | 1995-07-29 |
| Chirpy   | Gwen   | bird    | f    | 1998-09-11 | NULL       |
| Whistler | Gwen   | bird    | NULL | 1997-12-09 | NULL       |
| Slim     | Benny  | snake   | m    | 1996-04-29 | NULL       |
| Puffball | Diane  | hamster | f    | 1999-03-30 | NULL       |
+----------+--------+---------+------+------------+------------+

This form of SELECT is useful if you want to review your entire table, for instance, after you've just loaded it with your initial dataset. As it happens, the output just shown reveals an error in your data file: Bowser appears to have been born after he died! Consulting your original pedigree papers, you find that the correct birth year is 1989, not 1998.

There are are least a couple of ways to fix this:

As shown above, it is easy to retrieve an entire table. But typically you don't want to do that, particularly when the table becomes large. Instead, you're usually more interested in answering a particular question, in which case you specify some constraints on the information you want. Let's look at some selection queries in terms of questions about your pets that they answer.

8.4.4.2 Selecting particular rows

You can select only particular rows from your table. For example, if you want to verify the change that you made to Bowser's birth date, select Bowser's record like this:

mysql> SELECT * FROM pet WHERE name = "Bowser";
+--------+-------+---------+------+------------+------------+
| name   | owner | species | sex  | birth      | death      |
+--------+-------+---------+------+------------+------------+
| Bowser | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+-------+---------+------+------------+------------+

The output confirms that the year is correctly recorded now as 1989, not 1998.

String comparisons are normally case-insensitive, so you can specify the name as "bowser", "BOWSER", etc. The query result will be the same.

You can specify conditions on any column, not just name. For example, if you want to know which animals were born after 1998, test the birth column:

mysql> SELECT * FROM pet WHERE birth >= "1998-1-1";
+----------+-------+---------+------+------------+-------+
| name     | owner | species | sex  | birth      | death |
+----------+-------+---------+------+------------+-------+
| Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
| Puffball | Diane | hamster | f    | 1999-03-30 | NULL  |
+----------+-------+---------+------+------------+-------+

You can combine conditions, for example, to locate female dogs:

mysql> SELECT * FROM pet WHERE species = "dog" AND sex = "f";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

The preceding query uses the AND logical operator. There is also an OR operator:

mysql> SELECT * FROM pet WHERE species = "snake" OR species = "bird";
+----------+-------+---------+------+------------+-------+
| name     | owner | species | sex  | birth      | death |
+----------+-------+---------+------+------------+-------+
| Chirpy   | Gwen  | bird    | f    | 1998-09-11 | NULL  |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL  |
| Slim     | Benny | snake   | m    | 1996-04-29 | NULL  |
+----------+-------+---------+------+------------+-------+

AND and OR may be intermixed. If you do that, it's a good idea to use parentheses to indicate how conditions should be grouped:

mysql> SELECT * FROM pet WHERE (species = "cat" AND sex = "m")
    -> OR (species = "dog" AND sex = "f");
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

8.4.4.3 Selecting particular columns

If you don't want to see entire rows from your table, just name the columns in which you're interested, separated by commas. For example, if you want to know when your animals were born, select the name and birth columns:

mysql> SELECT name, birth FROM pet;
+----------+------------+
| name     | birth      |
+----------+------------+
| Fluffy   | 1993-02-04 |
| Claws    | 1994-03-17 |
| Buffy    | 1989-05-13 |
| Fang     | 1990-08-27 |
| Bowser   | 1989-08-31 |
| Chirpy   | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim     | 1996-04-29 |
| Puffball | 1999-03-30 |
+----------+------------+

To find out who owns pets, use this query:

mysql> SELECT owner FROM pet;
+--------+
| owner  |
+--------+
| Harold |
| Gwen   |
| Harold |
| Benny  |
| Diane  |
| Gwen   |
| Gwen   |
| Benny  |
| Diane  |
+--------+

However, notice that the query simply retrieves the owner field from each record, and some of them appear more than once. To minimize the output, retrieve each unique output record just once by adding the keyword DISTINCT:

mysql> SELECT DISTINCT owner FROM pet;
+--------+
| owner  |
+--------+
| Benny  |
| Diane  |
| Gwen   |
| Harold |
+--------+

You can use a WHERE clause to combine row selection with column selection. For example, to get birth dates for dogs and cats only, use this query:

mysql> SELECT name, species, birth FROM pet
    -> WHERE species = "dog" OR species = "cat";
+--------+---------+------------+
| name   | species | birth      |
+--------+---------+------------+
| Fluffy | cat     | 1993-02-04 |
| Claws  | cat     | 1994-03-17 |
| Buffy  | dog     | 1989-05-13 |
| Fang   | dog     | 1990-08-27 |
| Bowser | dog     | 1989-08-31 |
+--------+---------+------------+

8.4.4.4 Sorting rows

You may have noticed in the preceding examples that the result rows are displayed in no particular order. However, it's often easier to examine query output when the rows are sorted in some meaningful way. To sort a result, use an ORDER BY clause.

Here are animal birthdays, sorted by date:

mysql> SELECT name, birth FROM pet ORDER BY birth;
+----------+------------+
| name     | birth      |
+----------+------------+
| Buffy    | 1989-05-13 |
| Bowser   | 1989-08-31 |
| Fang     | 1990-08-27 |
| Fluffy   | 1993-02-04 |
| Claws    | 1994-03-17 |
| Slim     | 1996-04-29 |
| Whistler | 1997-12-09 |
| Chirpy   | 1998-09-11 |
| Puffball | 1999-03-30 |
+----------+------------+

To sort in reverse order, add the DESC (descending) keyword to the name of the column you are sorting by:

mysql> SELECT name, birth FROM pet ORDER BY birth DESC;
+----------+------------+
| name     | birth      |
+----------+------------+
| Puffball | 1999-03-30 |
| Chirpy   | 1998-09-11 |
| Whistler | 1997-12-09 |
| Slim     | 1996-04-29 |
| Claws    | 1994-03-17 |
| Fluffy   | 1993-02-04 |
| Fang     | 1990-08-27 |
| Bowser   | 1989-08-31 |
| Buffy    | 1989-05-13 |
+----------+------------+

You can sort on multiple columns. For example, to sort by type of animal, then by birth date within animal type with youngest animals first, use the following query:

mysql> SELECT name, species, birth FROM pet ORDER BY species, birth DESC;
+----------+---------+------------+
| name     | species | birth      |
+----------+---------+------------+
| Chirpy   | bird    | 1998-09-11 |
| Whistler | bird    | 1997-12-09 |
| Claws    | cat     | 1994-03-17 |
| Fluffy   | cat     | 1993-02-04 |
| Fang     | dog     | 1990-08-27 |
| Bowser   | dog     | 1989-08-31 |
| Buffy    | dog     | 1989-05-13 |
| Puffball | hamster | 1999-03-30 |
| Slim     | snake   | 1996-04-29 |
+----------+---------+------------+

Note that the DESC keyword applies only to the column name immediately preceding it (birth); species values are still sorted in ascending order.

8.4.4.5 Date calculations

MySQL provides several functions that you can use to perform calculations on dates, for example, to calculate ages or extract parts of dates.

To determine how many years old each of your pets is, compute age as the difference between the birth date and the current date. Do this by converting the two dates to days, take the difference, and divide by 365 (the number of days in a year):

mysql> SELECT name, (TO_DAYS(NOW())-TO_DAYS(birth))/365 FROM pet;
+----------+-------------------------------------+
| name     | (TO_DAYS(NOW())-TO_DAYS(birth))/365 |
+----------+-------------------------------------+
| Fluffy   |                                6.15 |
| Claws    |                                5.04 |
| Buffy    |                                9.88 |
| Fang     |                                8.59 |
| Bowser   |                                9.58 |
| Chirpy   |                                0.55 |
| Whistler |                                1.30 |
| Slim     |                                2.92 |
| Puffball |                                0.00 |
+----------+-------------------------------------+

Although the query works, there are some things about it that could be improved. First, the result could be scanned more easily if the rows were presented in some order. Second, the heading for the age column isn't very meaningful.

The first problem can be handled by adding an ORDER BY name clause to sort the output by name. To deal with the column heading, provide a name for the column so that a different label appears in the output (this is called a column alias):

mysql> SELECT name, (TO_DAYS(NOW())-TO_DAYS(birth))/365 AS age
    -> FROM pet ORDER BY name;
+----------+------+
| name     | age  |
+----------+------+
| Bowser   | 9.58 |
| Buffy    | 9.88 |
| Chirpy   | 0.55 |
| Claws    | 5.04 |
| Fang     | 8.59 |
| Fluffy   | 6.15 |
| Puffball | 0.00 |
| Slim     | 2.92 |
| Whistler | 1.30 |
+----------+------+

To sort the output by age rather than name, just use a different ORDER BY clause:

mysql>  SELECT name, (TO_DAYS(NOW())-TO_DAYS(birth))/365 AS age
    ->  FROM pet ORDER BY age;
+----------+------+
| name     | age  |
+----------+------+
| Puffball | 0.00 |
| Chirpy   | 0.55 |
| Whistler | 1.30 |
| Slim     | 2.92 |
| Claws    | 5.04 |
| Fluffy   | 6.15 |
| Fang     | 8.59 |
| Bowser   | 9.58 |
| Buffy    | 9.88 |
+----------+------+

A similar query can be used to determine age at death for animals that have died. You determine which animals these are by checking whether or not the death value is NULL. Then, for those with non-NULL values, compute the difference between the death and birth values:

mysql>  SELECT name, birth, death, (TO_DAYS(death)-TO_DAYS(birth))/365 AS age
    ->  FROM pet WHERE death IS NOT NULL ORDER BY age;
+--------+------------+------------+------+
| name   | birth      | death      | age  |
+--------+------------+------------+------+
| Bowser | 1989-08-31 | 1995-07-29 | 5.91 |
+--------+------------+------------+------+

The query uses death IS NOT NULL rather than death != NULL because NULL is a special value. This is explained later. See section 8.4.4.6 Working with NULL values.

What if you want to know which animals have birthdays next month? For this type of calculation, year and day are irrelevant, you simply want to extract the month part of the birth column. MySQL provides several date-part extraction functions, such as YEAR(), MONTH() and DAYOFMONTH(). MONTH() is the appropriate function here. To see how it works, run a simple query that displays the value of both birth and MONTH(birth):

mysql> SELECT name, birth, MONTH(birth) FROM pet;
+----------+------------+--------------+
| name     | birth      | MONTH(birth) |
+----------+------------+--------------+
| Fluffy   | 1993-02-04 |            2 |
| Claws    | 1994-03-17 |            3 |
| Buffy    | 1989-05-13 |            5 |
| Fang     | 1990-08-27 |            8 |
| Bowser   | 1989-08-31 |            8 |
| Chirpy   | 1998-09-11 |            9 |
| Whistler | 1997-12-09 |           12 |
| Slim     | 1996-04-29 |            4 |
| Puffball | 1999-03-30 |            3 |
+----------+------------+--------------+

Finding animals with birthdays in the upcoming month is easy, too. Suppose the current month is April. Then the month value is 4 and you look for animals born in May (month 5) like this:

mysql> SELECT name, birth FROM pet WHERE MONTH(birth) = 5;
+-------+------------+
| name  | birth      |
+-------+------------+
| Buffy | 1989-05-13 |
+-------+------------+

There is a small complication if the current month is December, of course. You don't just add one to the month number (12) and look for animals born in month 13, because there is no such month. Instead, you look for animals born in January (month 1).

You can even write the query so that it works no matter what the current month is. That way you don't have to use a particular month number in the query. DATE_ADD() allows you to add a time interval to a given date. If you add a month to the value of NOW(), then extract the month part with MONTH(), the result produces the month in which to look for birthdays:

mysql> SELECT name, birth FROM pet
    -> WHERE MONTH(birth) = MONTH(DATE_ADD(NOW(), INTERVAL 1 MONTH));

A different way to accomplish the same task is to add 1 to get the next month after the current one (after using the modulo function (MOD) to "wrap around" the month value to 0 if it is currently 12):

mysql> SELECT name, birth FROM pet
    -> WHERE MONTH(birth) = MOD(MONTH(NOW()),12) + 1;

8.4.4.6 Working with NULL values

The NULL value can be surprising until you get used to it. Conceptually, NULL means "missing value" or "unknown value" and it is treated somewhat differently than other values. To test for NULL, you cannot use the arithmetic comparison operators such as =, < or !=. To demonstrate this for yourself, try the following query:

mysql> SELECT 1 = NULL, 1 != NULL, 1 < NULL, 1 > NULL;
+----------+-----------+----------+----------+
| 1 = NULL | 1 != NULL | 1 < NULL | 1 > NULL |
+----------+-----------+----------+----------+
|     NULL |      NULL |     NULL |     NULL |
+----------+-----------+----------+----------+

Clearly you get no meaningful results from these comparisons. Use the IS NULL and IS NOT NULL operators instead:

mysql> SELECT 1 IS NULL, 1 IS NOT NULL;
+-----------+---------------+
| 1 IS NULL | 1 IS NOT NULL |
+-----------+---------------+
|         0 |             1 |
+-----------+---------------+

In MySQL, 0 means false and 1 means true.

This special treatment of NULL is why, in the previous section, it was necessary to determine which animals are no longer alive using death IS NOT NULL instead of death != NULL.

8.4.4.7 Pattern matching

MySQL provides standard SQL pattern matching as well as a form of pattern matching based on extended regular expressions similar to those used by Unix utilities such as vi, grep and sed.

SQL pattern matching allows you to use `_' to match any single character, and `%' to match an arbitrary number of characters (including zero characters). SQL patterns are case insensitive. Some examples are shown below. Note that you do not use = or != when you use SQL patterns; use the LIKE or NOT LIKE comparison operators instead.

To find names beginning with `b':

mysql> SELECT * FROM pet WHERE name LIKE "b%";
+--------+--------+---------+------+------------+------------+
| name   | owner  | species | sex  | birth      | death      |
+--------+--------+---------+------+------------+------------+
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
| Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+

To find names ending with `fy':

mysql> SELECT * FROM pet WHERE name LIKE "%fy";
+--------+--------+---------+------+------------+-------+
| name   | owner  | species | sex  | birth      | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
+--------+--------+---------+------+------------+-------+

To find names containing a `w':

mysql> SELECT * FROM pet WHERE name LIKE "%w%";
+----------+-------+---------+------+------------+------------+
| name     | owner | species | sex  | birth      | death      |
+----------+-------+---------+------+------------+------------+
| Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
| Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
+----------+-------+---------+------+------------+------------+

To find names containing exactly five characters, use the `_' pattern character:

mysql> SELECT * FROM pet WHERE name LIKE "_____";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

The other type of pattern matching provided by MySQL uses extended regular expressions. When you test for a match for this type of pattern, use the REGEXP and NOT REGEXP operators (or RLIKE and NOT RLIKE, which are synonyms).

Some characteristics of extended regular expressions are:

To demonstrate how extended regular expressions work, the LIKE queries shown above are rewritten below to use REGEXP:

To find names beginning with `b', use `^' to match the beginning of the name and `[bB]' to match either lowercase or uppercase `b':

mysql> SELECT * FROM pet WHERE name REGEXP "^[bB]";
+--------+--------+---------+------+------------+------------+
| name   | owner  | species | sex  | birth      | death      |
+--------+--------+---------+------+------------+------------+
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL       |
| Bowser | Diane  | dog     | m    | 1989-08-31 | 1995-07-29 |
+--------+--------+---------+------+------------+------------+

To find names ending with `fy', use `$' to match the end of the name:

mysql> SELECT * FROM pet WHERE name REGEXP "fy$";
+--------+--------+---------+------+------------+-------+
| name   | owner  | species | sex  | birth      | death |
+--------+--------+---------+------+------------+-------+
| Fluffy | Harold | cat     | f    | 1993-02-04 | NULL  |
| Buffy  | Harold | dog     | f    | 1989-05-13 | NULL  |
+--------+--------+---------+------+------------+-------+

To find names containing a `w', use `[wW]' to match either lowercase or uppercase `w':

mysql> SELECT * FROM pet WHERE name REGEXP "[wW]";
+----------+-------+---------+------+------------+------------+
| name     | owner | species | sex  | birth      | death      |
+----------+-------+---------+------+------------+------------+
| Claws    | Gwen  | cat     | m    | 1994-03-17 | NULL       |
| Bowser   | Diane | dog     | m    | 1989-08-31 | 1995-07-29 |
| Whistler | Gwen  | bird    | NULL | 1997-12-09 | NULL       |
+----------+-------+---------+------+------------+------------+

Since a regular expression pattern matches if it occurs anywhere in the value, it is not necessary in the previous query to put a wildcard on either side of the pattern to get it to match the entire value like it would be if you used an SQL pattern.

To find names containing exactly five characters, use `^' and `$' to match the beginning and end of the name, and five instances of `.' in between:

mysql> SELECT * FROM pet WHERE name REGEXP "^.....$";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

You could also write the previous query using the `{n}' "repeat-n-times" operator:

mysql> SELECT * FROM pet WHERE name REGEXP "^.{5}$";
+-------+--------+---------+------+------------+-------+
| name  | owner  | species | sex  | birth      | death |
+-------+--------+---------+------+------------+-------+
| Claws | Gwen   | cat     | m    | 1994-03-17 | NULL  |
| Buffy | Harold | dog     | f    | 1989-05-13 | NULL  |
+-------+--------+---------+------+------------+-------+

8.4.4.8 Counting rows

Databases are often used to answer the question, "how often does a certain type of data occur in a table?" For example, you might want to know how many pets you have, or how many pets each owner has, or you might want to perform various kinds of censuses on your animals.

Counting the total number of animals you have is the same question as "how many rows are in the pet table?," since there is one record per pet. The COUNT() function counts the number of non-NULL results, so the query to count your animals looks like this:

mysql> SELECT COUNT(*) FROM pet;
+----------+
| COUNT(*) |
+----------+
|        9 |
+----------+

Earlier, you retrieved the names of the people who owned pets. You can use COUNT() if you want to find out how many pets each owner has:

mysql> SELECT owner, COUNT(*) FROM pet GROUP BY owner;
+--------+----------+
| owner  | COUNT(*) |
+--------+----------+
| Benny  |        2 |
| Diane  |        2 |
| Gwen   |        3 |
| Harold |        2 |
+--------+----------+

Note the use of GROUP BY to group together all records for each owner. Without it, all you get is an error message:

mysql> SELECT owner, COUNT(owner) FROM pet;
ERROR 1140 at line 1: Mixing of GROUP columns (MIN(),MAX(),COUNT()...)
with no GROUP columns is illegal if there is no GROUP BY clause

COUNT() and GROUP BY are useful for characterizing your data in various ways. The following examples show different ways to perform animal census operations.

Number of animals per species:

mysql> SELECT species, COUNT(*) FROM pet GROUP BY species;
+---------+----------+
| species | COUNT(*) |
+---------+----------+
| bird    |        2 |
| cat     |        2 |
| dog     |        3 |
| hamster |        1 |
| snake   |        1 |
+---------+----------+

Number of animals per sex:

mysql> SELECT sex, COUNT(*) FROM pet GROUP BY sex;
+------+----------+
| sex  | COUNT(*) |
+------+----------+
| NULL |        1 |
| f    |        4 |
| m    |        4 |
+------+----------+

(In this output, NULL indicates "sex unknown.")

Number of animals per combination of species and sex:

mysql> SELECT species, sex, COUNT(*) FROM pet GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | NULL |        1 |
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

You need not retrieve an entire table when you use COUNT(). For example, the previous query, when performed just on dogs and cats, looks like this:

mysql> SELECT species, sex, COUNT(*) FROM pet
    -> WHERE species = "dog" OR species = "cat"
    -> GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
+---------+------+----------+

Or, if you wanted the number of animals per sex only for known-sex animals:

mysql> SELECT species, sex, COUNT(*) FROM pet
    -> WHERE sex IS NOT NULL
    -> GROUP BY species, sex;
+---------+------+----------+
| species | sex  | COUNT(*) |
+---------+------+----------+
| bird    | f    |        1 |
| cat     | f    |        1 |
| cat     | m    |        1 |
| dog     | f    |        1 |
| dog     | m    |        2 |
| hamster | f    |        1 |
| snake   | m    |        1 |
+---------+------+----------+

8.4.5 Using more than one table

The pet table keeps track of which pets you have. If you want to record other information about them, such as events in their lives like visits to the vet or when litters are born, you need another table. What should this table look like?

Given these considerations, the CREATE TABLE statement for the event table might look like this:

mysql> CREATE TABLE event (name VARCHAR(20), date DATE,
    -> type VARCHAR(15), remark VARCHAR(255));

As with the pet table, it's easiest to load the initial records by creating a tab-delimited text file containing the information:

Fluffy 1995-05-15 litter 4 kittens, 3 female, 1 male
Buffy 1993-06-23 litter 5 puppies, 2 female, 3 male
Buffy 1994-06-19 litter 3 puppies, 3 female
Chirpy 1999-03-21 vet needed beak straightened
Slim 1997-08-03 vet broken rib
Bowser 1991-10-12 kennel
Fang 1991-10-12 kennel
Fang 1998-08-28 birthday Gave him a new chew toy
Claws 1998-03-17 birthday Gave him a new flea collar
Whistler 1998-12-09 birthday First birthday

Load the records like this:

mysql> LOAD DATA LOCAL INFILE "event.txt" INTO TABLE event;

Based on what you've learned from the queries you've run on the pet table, you should be able to perform retrievals on the records in the event table; the principles are the same. But when is the event table by itself insufficient to answer questions you might ask?

Suppose you want to find out the ages of each pet when they had their litters. The event table indicates when this occurred, but to calculate age of the mother, you need her birth date. Since that is stored in the pet table, you need both tables for the query:

mysql> SELECT pet.name, (TO_DAYS(date) - TO_DAYS(birth))/365 AS age, remark
    -> FROM pet, event
    -> WHERE pet.name = event.name AND type = "litter";
+--------+------+-----------------------------+
| name   | age  | remark                      |
+--------+------+-----------------------------+
| Fluffy | 2.27 | 4 kittens, 3 female, 1 male |
| Buffy  | 4.12 | 5 puppies, 2 female, 3 male |
| Buffy  | 5.10 | 3 puppies, 3 female         |
+--------+------+-----------------------------+

There are several things to note about this query:

You need not have two different tables to perform a join. Sometimes it is useful to join a table to itself, if you want to compare records in a table to other records in that same table. For example, to find breeding pairs among your pets, you can join the pet table with itself to pair up males and females of like species:

mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species
    -> FROM pet AS p1, pet AS p2
    -> WHERE p1.species = p2.species AND p1.sex = "f" AND p2.sex = "m";
+--------+------+--------+------+---------+
| name   | sex  | name   | sex  | species |
+--------+------+--------+------+---------+
| Fluffy | f    | Claws  | m    | cat     |
| Buffy  | f    | Fang   | m    | dog     |
| Buffy  | f    | Bowser | m    | dog     |
+--------+------+--------+------+---------+

In this query, we specify aliases for the table name in order to be able to refer to the columns and keep straight which instance of the table each column reference is associated with.

8.5 Getting information about databases and tables

What if you forget the name of a database or table, or what the structure of a given table is (e.g., what its columns are called)? MySQL addresses this problem through several statements that provide information about the databases and tables it supports.

You have already seen SHOW DATABASES, which lists the databases managed by the server. To find out which database is currently selected, use the DATABASE() function:

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| menagerie  |
+------------+

If you haven't selected any database yet, the result is blank.

To find out what tables the current database contains (for example, when you're not sure about the name of a table), use this command:

mysql> SHOW TABLES;
+---------------------+
| Tables in menagerie |
+---------------------+
| event               |
| pet                 |
+---------------------+

If you want to find out about the structure of a table, the DESCRIBE command is useful; it displays information about each of a table's columns:

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(20) | YES  |     | NULL    |       |
| owner   | varchar(20) | YES  |     | NULL    |       |
| species | varchar(20) | YES  |     | NULL    |       |
| sex     | char(1)     | YES  |     | NULL    |       |
| birth   | date        | YES  |     | NULL    |       |
| death   | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

Field indicates the column name, Type is the data type for the column, Null indicates whether or not the column can contain NULL values, Key indicates whether or not the column is indexed and Default specifies the column's default value.

If you have indexes on a table, SHOW INDEX FROM tbl_name produces information about them.

8.6 Using mysql in batch mode

In the previous sections, you used mysql interactively to enter queries and view the results. You can also run mysql in batch mode. To do this, put the commands you want to run in a file, then tell mysql to read its input from the file:

shell> mysql < batch-file

If you need to specify connection parameters on the command line, the command might look like this:

shell> mysql -h host -u user -p < batch-file
Enter password: ********

When you use mysql this way, you are creating a script file, then executing the script.

Why use a script? Here are a few reasons:

The default output format is different (more concise) when you run mysql in batch mode than when you use it interactively. For example, the output of SELECT DISTINCT species FROM pet looks like this when run interactively:

+---------+
| species |
+---------+
| bird    |
| cat     |
| dog     |
| hamster |
| snake   |
+---------+

But like this when run in batch mode:

species
bird
cat
dog
hamster
snake

If you want to get the interactive output format in batch mode, use mysql -t. To echo to the output the commands that are executed, use mysql -vvv.

8.7 Queries from twin project

At Analytikerna and Lentus, we have been doing the systems and field work for a big research project. This project is a collaboration between the Institute of Environmental Medicine at Karolinska Institutet Stockholm and the Section on Clinical Research in Aging and Psychology at the University of Southern California.

The project involves a screening part where all twins in Sweden older than 65 years are interviewed by telephone. Twins who meet certain criteria are passed on to the next stage. In this latter stage, twins who want to participate are visited by a doctor/nurse team. Some of the examinations include physical and neuropsychological examination, laboratory testing, neuroimaging, psychological status assessment, and family history collection. In addition, data are collected on medical and environmental risk factors.

More information about Twin studies can be found at:

http://www.imm.ki.se/TWIN/TWINUKW.HTM

The latter part of the project is administered with a web interface written using Perl and MySQL.

Each night all data from the interviews are moved into a MySQL database.

8.7.1 Find all non-distributed twins

The following query is used to determine who goes into the second part of the project:

select
        concat(p1.id, p1.tvab) + 0 as tvid,
        concat(p1.christian_name, " ", p1.surname) as Name,
        p1.postal_code as Code,
        p1.city as City,
        pg.abrev as Area,
        if(td.participation = "Aborted", "A", " ") as A,
        p1.dead as dead1,
        l.event as event1,
        td.suspect as tsuspect1,
        id.suspect as isuspect1,
        td.severe as tsevere1,
        id.severe as isevere1,
        p2.dead as dead2,
        l2.event as event2,
        h2.nurse as nurse2,
        h2.doctor as doctor2,
        td2.suspect as tsuspect2,
        id2.suspect as isuspect2,
        td2.severe as tsevere2,
        id2.severe as isevere2,
        l.finish_date
from
        twin_project as tp
        /* For Twin 1 */
        left join twin_data as td on tp.id = td.id and tp.tvab = td.tvab
        left join informant_data as id on tp.id = id.id and tp.tvab = id.tvab
        left join harmony as h on tp.id = h.id and tp.tvab = h.tvab
        left join lentus as l on tp.id = l.id and tp.tvab = l.tvab
        /* For Twin 2 */
        left join twin_data as td2 on p2.id = td2.id and p2.tvab = td2.tvab
        left join informant_data as id2 on p2.id = id2.id and p2.tvab = id2.tvab
        left join harmony as h2 on p2.id = h2.id and p2.tvab = h2.tvab
        left join lentus as l2 on p2.id = l2.id and p2.tvab = l2.tvab,
        person_data as p1,
        person_data as p2,
        postal_groups as pg
where
        /* p1 gets main twin and p2 gets his/her twin. */
        /* ptvab is a field inverted from tvab */
        p1.id = tp.id and p1.tvab = tp.tvab and
        p2.id = p1.id and p2.ptvab = p1.tvab and
        /* Just the sceening survey */
        tp.survey_no = 5 and
        /* Skip if partner died before 65 but allow emigration (dead=9) */
        (p2.dead = 0 or p2.dead = 9 or
         (p2.dead = 1 and
          (p2.death_date = 0 or
           (((to_days(p2.death_date) - to_days(p2.birthday)) / 365)
            >= 65))))
        and
        (
        /* Twin is suspect */
        (td.future_contact = 'Yes' and td.suspect = 2) or
        /* Twin is suspect - Informant is Blessed */
        (td.future_contact = 'Yes' and td.suspect = 1 and id.suspect = 1) or
        /* No twin - Informant is Blessed */
        (ISNULL(td.suspect) and id.suspect = 1 and id.future_contact = 'Yes') or
        /* Twin broken off - Informant is Blessed */
        (td.participation = 'Aborted'
         and id.suspect = 1 and id.future_contact = 'Yes') or
        /* Twin broken off - No inform - Have partner */
        (td.participation = 'Aborted' and ISNULL(id.suspect) and p2.dead = 0))
        and
        l.event = 'Finished'
        /* Get at area code */
        and substring(p1.postal_code, 1, 2) = pg.code
        /* Not already distributed */
        and (h.nurse is NULL or h.nurse=00 or h.doctor=00)
        /* Has not refused or been aborted */
        and not (h.status = 'Refused' or h.status = 'Aborted'
        or h.status = 'Died' or h.status = 'Other')
order by
        tvid;

Some explanations:

concat(p1.id, p1.tvab) + 0 as tvid
We want to sort on the concatenated id and tvab in numerical order. Adding 0 to the result causes MySQL to treat the result as a number.
column id
This identifies a pair of twins. It is a key in all tables.
column tvab
This identifies a twin in a pair. It has a value of 1 or 2.
column ptvab
This is an inverse of tvab. When tvab is 1 this is 2, and vice versa. It exists to save typing and to make it easier for MySQL to optimize the query.

This query demonstrates, among other things, how to do lookups on a table from the same table with a join (p1 and p2). In the example, this is used to check whether a twin's partner died before the age of 65. If so, the row is not returned.

All of the above exist in all tables with twin-related information. We have a key on both id,tvab (all tables) and id,ptvab (person_data) to make queries faster.

On our production machine (A 200MHz UltraSPARC), this query returns about 150-200 rows and takes less than one second.

The current number of records in the tables used above:
Table Rows
person_data 71074
lentus 5291
twin_project 5286
twin_data 2012
informant_data 663
harmony 381
postal_groups 100

8.7.2 Show a table on twin pair status

Each interview ends with a status code called event. The query shown below is used to display a table over all twin pairs combined by event. This indicates in how many pairs both twins are finished, in how many pairs one twin is finished and the other refused, and so on.

select
        t1.event,
        t2.event,
        count(*)
from
        lentus as t1,
        lentus as t2,
        twin_project as tp
where
        /* We are looking at one pair at a time */
        t1.id = tp.id
        and t1.tvab=tp.tvab
        and t1.id = t2.id
        /* Just the sceening survey */
        and tp.survey_no = 5
        /* This makes each pair only appear once */
        and t1.tvab='1' and t2.tvab='2'
group by
        t1.event, t2.event;

9 MySQL server functions

9.1 What languages are supported by MySQL?

mysqld can issue error messages in the following languages: Czech, Dutch, English (the default), Estonia, French, German, Hungarian, Italian, Norwegian, Norwegian-ny, Polish, Portuguese, Spanish and Swedish.

To start mysqld with a particular language, use either the --language=lang or -L lang options. For example:

shell> mysqld --language=swedish

or:

shell> mysqld --language=/usr/local/share/swedish

Note that all language names are specified in lowercase.

The language files are located (by default) in `mysql_base_dir/share/LANGUAGE/'.

To update the error message file, you should edit the `errmsg.txt' file and execute the following command to generate the `errmsg.sys' file:

shell> comp_err errmsg.txt errmsg.sys

If you upgrade to a newer version of MySQL, remember to repeat your changes with the new `errmsg.txt' file.

9.1.1 The character set used for data and sorting

By default, MySQL uses the ISO-8859-1 (Latin1) character set. This is the character set used in the USA and western Europe.

The character set determines what characters are allowed in names and how things are sorted by the ORDER BY and GROUP BY clauses of the SELECT statement.

You can change the character set at compile time by using the --with-charset=charset option to configure. See section 4.7.1 Quick installation overview.

To add another character set to MySQL, use the following procedure:

9.1.2 Adding a new character set

  1. Choose a name for the character set, denoted MYSET below.
  2. Create the file `strings/ctype-MYSET.c' in the MySQL source distribution.
  3. Look at one of the existing `ctype-*.c' files to see what needs to be defined. Note that the arrays in your file must have names like ctype_MYSET, to_lower_MYSET and so on. to_lower[] and to_upper[] are simple arrays that hold the lowercase and uppercase characters corresponding to each member of the character set. For example:
    to_lower['A'] should contain 'a'
    to_upper['a'] should contain 'A'
    
    sort_order[] is a map indicating how characters should be ordered for comparison and sorting purposes. For many character sets, this is the same as to_upper[] (which means sorting will be case insensitive). MySQL will sort characters based on the value of sort_order[character]. ctype[] is an array of bit values, with one element for one character. (Note that to_lower[], to_upper[] and sort_order[] are indexed by character value, but ctype[] is indexed by character value + 1. This is an old legacy to be able to handle EOF.) You can find the following bitmask definitions in `m_ctype.h':
    #define _U      01      /* Upper case */
    #define _L      02      /* Lower case */
    #define _N      04      /* Numeral (digit) */
    #define _S      010     /* Spacing character */
    #define _P      020     /* Punctuation */
    #define _C      040     /* Control character */
    #define _B      0100    /* Blank */
    #define _X      0200    /* heXadecimal digit */
    
    The ctype[] entry for each character should be the union of the applicable bitmask values that describe the character. For example, 'A' is an uppercase character (_U) as well as a hexadecimal digit (_X), so ctype['A'+1] should contain the value:
    _U + _X = 01 + 0200 = 0201
    
  4. Add a unique number for your character set to `include/m_ctype.h.in'.
  5. Add the character set name to the CHARSETS_AVAILABLE list in configure.in.
  6. Reconfigure, recompile and test.

9.1.3 Multi-byte character support

If you are creating a multi-byte character set, you can use the _MB macros. In `include/m_ctype.h.in', add:

#define MY_CHARSET_MYSET  X
#if MY_CHARSET_CURRENT == MY_CHARSET_MYSET
#define USE_MB
#define USE_MB_IDENT
#define ismbchar(p, end)  (...)
#define ismbhead(c)       (...)
#define mbcharlen(c)      (...)
#define MBMAXLEN          N
#endif

Where:

MY_CHARSET_MYSET A unique character set value.
USE_MB This character set has multi-byte characters, handled by ismbhead() and mbcharlen()
USE_MB_IDENT (optional) If defined, you can use table and column names that use multi-byte characters
ismbchar(p, e) return 0 if p is not a multi-byte character string, or the size of the character (in bytes) if it is. p and e point to the beginning and end of the string. Check from (char*)p to (char*)e-1.
ismbhead(c) True if c is the first character of a multi-byte character string
mbcharlen(c) Size of a multi-byte character string if c is the first character of such a string
MBMAXLEN Size in bytes of the largest character in the set

9.2 The update log

When started with the --log-update=file_name option, mysqld writes a log file containing all SQL commands that update data. The file is written in the data directory and has a name of file_name.#, where # is a number that is incremented each time you execute mysqladmin refresh or mysqladmin flush-logs, the FLUSH LOGS statement, or restart the server.

If you use the --log or -l options, the filename is `hostname.log', and restarts and refreshes do not cause a new log file to be generated. By default, the mysql.server script starts the MySQL server with the -l option. If you need better performance when you start using MySQL in a production environment, you can remove the -l option from mysql.server.

Update logging is smart since it logs only statements that really update data. So an UPDATE or a DELETE with a WHERE that finds no rows is not written to the log. It even skips UPDATE statements that set a column to the value it already has.

If you want to update a database from update log files, you could do the following (assuming your log files have names of the form `file_name.#'):

shell> ls -1 -t -r file_name.[0-9]* | xargs cat | mysql

ls is used to get all the log files in the right order.

This can be useful if you have to revert to backup files after a crash and you want to redo the updates that occurred between the time of the backup and the crash.

You can also use the update logs when you have a mirrored database on another host and you want to replicate the changes that have been made to the master database.

9.3 How big MySQL tables can be

MySQL itself has a 4G limit on table size, and operating systems have their own file size limits. On Linux, the current limit is 2G; on Solaris 2.5.1, the limit is 4G; on Solaris 2.6, the limit is going to be 1000G. Currently, table sizes are limited to either 4G (the MySQL limit) or the operating system limit, whichever is smaller. To get more than 4G requires some changes to MySQL that are on the TODO. See section F List of things we want to add to MySQL in the future (The TODO).

If your big table is going to be read-only, you could use pack_isam to merge and compress many tables to one. pack_isam usually compresses a table by at least 50%, so you can have, in effect, much bigger tables. See section 12.5 The MySQL compressed read-only table generator.

Another solution can be the included MERGE library, which allows you to handle a collection of identical tables as one. (Identical in this case means that all tables are created with identical column information.) Currently MERGE can only be used to scan a collection of tables because it doesn't support indexes. We will add indexes to this in the near future.

10 Getting maximum performance from MySQL

10.1 Tuning server parameters

You can get the default buffer sizes used by the mysqld server with this command:

shell> mysqld --help

This command produces a list of all mysqld options and configurable variables. The output includes the default values and looks something like this:

Possible variables for option --set-variable (-O) are:
back_log              current value: 5
connect_timeout       current value: 5
delayed_insert_timeout  current value: 300
delayed_insert_limit  current value: 100
delayed_queue_size    current value: 1000
flush_time            current value: 0
join_buffer_size      current value: 131072
key_buffer_size       current value: 1048540
long_query_time       current value: 10
max_allowed_packet    current value: 1048576
max_connections       current value: 100
max_connect_errors    current value: 10
max_delayed_threads   current value: 20
max_heap_table_size   current value: 16777216
max_join_size         current value: 4294967295
max_sort_length       current value: 1024
max_tmp_tables        current value: 32
net_buffer_length     current value: 16384
record_buffer         current value: 131072
sort_buffer           current value: 2097116
table_cache           current value: 64
tmp_table_size        current value: 1048576
thread_stack          current value: 131072
wait_timeout          current value: 28800

If there is a mysqld server currently running, you can see what values it actually is using for the variables by executing this command:

shell> mysqladmin variables

Each option is described below. Values for buffer sizes, lengths and stack sizes are given in bytes. You can specify values with a suffix of `K' or `M' to indicate kilobytes or megabytes. For example, 16M indicates 16 megabytes. Case of suffix letters does not matter; 16M and 16m are equivalent.

back_log
The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets VERY many connection requests in a very short time. It then takes some time (although very little) for the main thread to check the connection and start a new thread. The back_log value indicates how many requests can be stacked during this short time before MySQL momentarily stops answering new requests. You need to increase this only if you expect a large number of connections in a short period of time. In other words, this value is the size of the listen queue for incoming TCP/IP connections. Your operating system has its own limit on the size of this queue. The manual page for the Unix listen(2) system call should have more details. Check your OS documentation for the maximum value for this variable. Attempting to set back_log higher than this maximum will be ineffective.
connect_timeout
The number of seconds the mysqld server is waiting for a connect packet before responding with Bad handshake.
delayed_insert_timeout
How long a INSERT DELAYED thread should wait for INSERT statements before terminating.
delayed_insert_limit
After inserting delayed_insert_limit rows, the INSERT DELAYED handler will check if there are any SELECT statements pending. If so, it allows these to execute before continuing.
delayed_queue_size
How big a queue (in rows) should be allocated for handling INSERT DELAYED. If the queue becomes full, any client that does INSERT DELAYED will wait until there is room in the queue again.
flush_time
If this is set, then every flush_time seconds all tables will be closed (to free up resources and sync things to disk).
join_buffer
The size of the buffer that is used for full joins (joins that do not use indexes). The buffer is allocated one time for each full join between two tables. Increase this value to get a faster full join when adding indexes is not possible. (Normally the best way to get fast joins is to add indexes.)
key_buffer
Index blocks are buffered and are shared by all threads. key_buffer is the size of the buffer used for index blocks. You might want to increase this value when doing many DELETE or INSERT operations on a table with lots of indexes. To get even more speed, use LOCK TABLES. See section 7.23 LOCK TABLES/UNLOCK TABLES syntax.
long_query_time
If a query takes longer than this (in seconds), the slow_queries counter will be incremented.
max_allowed_packet
The maximum size of one packet. The message buffer is initialized to net_buffer_length bytes, but can grow up to max_allowed_packet bytes when needed. This value by default is small to catch big (possibly wrong) packets. You must increase this value if you are using big BLOB columns. It should be as big as the biggest BLOB you want to use.
max_connections
The number of simultaneous clients allowed. Increasing this value increases the number of file descriptors that mysqld requires. See below for comments on file descriptor limits.
max_connect_errors
If there is more than this number of interrupted connections from a host this host will be blocked for further connections. You can unblock a host with the command FLUSH HOSTS.
max_delayed_threads Don't start more than this number of
threads to handle INSERT DELAYED. If you try to insert data in a new table after all INSERT DELAYED threads are in use, the row will be inserted as if the DELAYED attribute wasn't specified.
max_join_size
Joins that are probably going to read more than max_join_size records return an error. Set this value if your users tend to perform joins without a WHERE clause that take a long time and return millions of rows.
max_sort_length
The number of bytes to use when sorting BLOB or TEXT values (only the first max_sort_length bytes of each value are used; the rest are ignored).
max_tmp_tables
(This option doesn't yet do anything). Maximum number of temporary tables a client can keep open at the same time.
net_buffer_length
The communication buffer is reset to this size between queries. This should not normally be changed, but if you have very little memory, you can set it to the expected size of a query. (That is, the expected length of SQL statements sent by clients. If statements exceed this length, the buffer is automatically enlarged, up to max_allowed_packet bytes.)
record_buffer
Each thread that does a sequential scan allocates a buffer of this size for each table it scans. If you do many sequential scans, you may want to increase this value.
sort_buffer
Each thread that needs to do a sort allocates a buffer of this size. Increase this value for faster ORDER BY or GROUP BY operations. See section 18.5 Where MySQL stores temporary files.
table_cache
The number of open tables for all threads. Increasing this value increases the number of file descriptors that mysqld requires. MySQL needs two file descriptors for each unique open table. See below for comments on file descriptor limits. For information about how the table cache works, see section 10.8 How MySQL opens and closes tables.
tmp_table_size
If a temporary table exceeds this size, MySQL generates an error of the form The table tbl_name is full. Increase the value of tmp_table_size if you do many advanced GROUP BY queries.
thread_stack
The stack size for each thread. Many of the limits detected by the crash-me test are dependent on this value. The default is normally large enough. See section 11 The MySQL benchmark suite.
wait_timeout
The number of seconds the server waits for activity on a connection before closing it.

table_cache, max_connections and max_tmp_tables affect the maximum number of files the server keeps open. If you increase one or both of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. However, you can increase the limit on many systems. Consult your OS documentation to find out how to do this, because the method for changing the limit varies widely from system to system.

table_cache is related to max_connections. For example, for 200 open connections, you should have a table cache of at least 200 * n, where n is the maximum number of tables in a join.

MySQL uses algorithms that are very scalable, so you can usually run with very little memory or give MySQL more memory to get better performance.

If you have much memory and many tables and want maximum performance with a moderate number of clients, you should use something like this:

shell> safe_mysqld -O key_buffer=16M -O table_cache=128 \
           -O sort_buffer=4M -O record_buffer=1M &

If you have little memory and lots of connections, use something like this:

shell> safe_mysqld -O key_buffer=512k -O sort_buffer=100k \
           -O record_buffer=100k &

or even:

shell> safe_mysqld -O key_buffer=512k -O sort_buffer=16k \
           -O table_cache=32 -O record_buffer=8k -O net_buffer=1K &

If there are very many connections, "swapping problems" may occur unless mysqld has been configured to use very little memory for each connection. mysqld performs better if you have enough memory for all connections, of course.

Note that if you change an option to mysqld, it remains in effect only for that instance of the server.

To see the effects of a parameter change, do something like this:

shell> mysqld -O key_buffer=32m --help

Make sure that the --help option is last; otherwise, the effect of any options listed after it on the command line will not be reflected in the output.

10.2 How MySQL uses memory

The list below indicates some of the ways that the mysqld server uses memory. Where applicable, the name of the server variable relevant to the memory use is given.

ps and other system status programs may report that mysqld uses a lot of memory. This may be caused by thread-stacks on different memory addresses. For example, the Solaris version of ps counts the unused memory between stacks as used memory. You can verify this by checking available swap with swap -s. We have tested mysqld with commercial memory-leakage detectors, so there should be no memory leaks.

10.3 How compiling and linking affects the speed of MySQL

Most of the following tests are done on Linux and with the MySQL benchmarks, but they should give some indication for other operating systems.

You get the fastest executable when you link with -static. Using Unix sockets rather than TCP/IP to connect to a database also gives better performance.

On Linux, you will get the fastest code when compiling with pgcc and -O6. To compile `sql_yacc.cc' with these options, you need 180M memory because gcc/pgcc needs a lot of memory to make all functions inline. You should also set CXX=gcc when configuring MySQL to avoid inclusion of the libstdc++ library.

The MySQL-Linux distribution provided by TcX is compiled with pgcc and linked statically.

10.4 How MySQL uses indexes

All indexes (PRIMARY, UNIQUE and INDEX()) are stored in B-trees. Strings are automatically prefix- and end-space compressed. See section 7.26 CREATE INDEX syntax.

Indexes are used to:

Suppose you issue the following SELECT statement:

mysql> SELECT * FROM tbl_name WHERE col1=val1 AND col2=val2;

If a multiple-column index exists on col1 and col2, the appropriate rows can be fetched directly. If separate single-column indexes exist on col1 and col2, the optimizer decides which index will find fewer rows and uses that index to fetch the rows.

If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to find rows. For example, if you have a three-column index on (col1,col2,col3), you have indexed search capabilities on (col1), (col1,col2) and (col1,col2,col3).

MySQL can't use a partial index if the columns don't form a leftmost prefix of the index. Suppose you have the SELECT statements shown below:

mysql> SELECT * FROM tbl_name WHERE col1=val1;
mysql> SELECT * FROM tbl_name WHERE col2=val2;
mysql> SELECT * FROM tbl_name WHERE col2=val2 AND col3=val3;

If an index exists on (col1,col2,col3), only the first query shown above uses the index. The second and third queries do involve indexed columns, but (col2) and (col2,col3) are not leftmost prefixes of (col1,col2,col3).

MySQL also uses indexes for LIKE comparisons if the argument to LIKE is a constant string that doesn't start with a wildcard character. For example, the following SELECT statements use indexes:

mysql> select * from tbl_name where key_col LIKE "Patrick%";
mysql> select * from tbl_name where key_col LIKE "Pat%_ck%";

In the first statement, only rows with "Patrick" <= key_col < "Patricl" are considered. In the second statement, only rows with "Pat" <= key_col < "Pau" are considered.

The following SELECT statements will not use indexes:

mysql> select * from tbl_name where key_col LIKE "%Patrick%";
mysql> select * from tbl_name where key_col LIKE other_col;

In the first statement, the LIKE value begins with a wildcard character. In the second statement, the LIKE value is not a constant.

Searching using column_name IS NULL will use indexes if column_name is a index.

MySQL normally uses the index that finds least number of rows. An index is used for columns that you compare with the following operators: =, >, >=, <, <=, BETWEEN and a LIKE with a non-wildcard prefix like 'something%'.

Any index that doesn't span all AND levels in the WHERE clause is not used to optimize the query.

The following WHERE clauses use indexes:

... WHERE index_part1=1 AND index_part2=2
... WHERE index=1 OR A=10 AND index=2      /* index = 1 OR index = 2 */
... WHERE index_part1='hello' AND index_part_3=5
          /* optimized like "index_part1='hello'" */

These WHERE clauses do NOT use indexes:

... WHERE index_part2=1 AND index_part3=2  /* index_part_1 is not used */
... WHERE index=1 OR A=10                  /* No index */
... WHERE index_part1=1 OR index_part2=10  /* No index spans all rows */

10.5 How MySQL optimizes WHERE clauses

(This section is incomplete; MySQL does many optimizations.)

In general, when you want to make a slow SELECT ... WHERE faster, the first thing to check is whether or not you can add an index. All references between different tables should usually be done with indexes. You can use the EXPLAIN command to determine which indexes are used for a SELECT. See section 7.21 EXPLAIN syntax (Get information about a SELECT).

Some of the optimizations performed by MySQL are listed below:

Some examples of queries that are very fast:

mysql> SELECT COUNT(*) FROM tbl_name;
mysql> SELECT MIN(key_part1),MAX(key_part1) FROM tbl_name;
mysql> SELECT MAX(key_part2) FROM tbl_name
           WHERE key_part_1=constant;
mysql> SELECT ... FROM tbl_name
           ORDER BY key_part1,key_part2,... LIMIT 10;
mysql> SELECT ... FROM tbl_name
           ORDER BY key_part1 DESC,key_part2 DESC,... LIMIT 10;

The following queries are resolved using only the index tree (assuming the indexed columns are numeric):

mysql> SELECT key_part1,key_part2 FROM tbl_name WHERE key_part1=val;
mysql> SELECT COUNT(*) FROM tbl_name
           WHERE key_part1=val1 AND key_part2=val2;
mysql> SELECT key_part2 FROM tbl_name GROUP BY key_part1;

The following queries use indexing to retrieve the rows in sorted order without a separate sorting pass:

mysql> SELECT ... FROM tbl_name ORDER BY key_part1,key_part2,...
mysql> SELECT ... FROM tbl_name ORDER BY key_part1 DESC,key_part2 DESC,...

10.6 How MySQL optimizes LEFT JOIN

A LEFT JOIN B is in MySQL implemented as follows

10.7 How MySQL optimizes LIMIT

In some cases MySQL will handle the query differently when you are using LIMIT # and not using HAVING:

10.8 How MySQL opens and closes tables

The cache of open tables can grow to a maximum of table_cache (default 64; this can be changed with with the -O table_cache=# option to mysqld). A table is never closed, except when the cache is full and another thread tries to open a table or if you use mysqladmin refresh or mysqladmin flush-tables.

When the table cache fills up, the server uses the following procedure to locate a cache entry to use:

A table is opened for each concurrent access. This means that if you have two threads accessing the same table or access the table twice in the same query (with AS) the table needs to be opened twice. The first open of any table takes two file descriptors; each additional use of the table takes only one file descriptor. The extra descriptor for the first open is used for the index file; this descriptor is shared among all threads.

10.8.1 Drawbacks of creating large numbers of tables in a database

If you have many files in a directory, open, close and create operations will be slow. If you execute SELECT statements on many different tables, there will be a little overhead when the table cache is full, because for every table that has to be opened, another must be closed. You can reduce this overhead by making the table cache larger.

10.9 Why so many open tables?

When you run mysqladmin status, you'll see something like this:

Uptime: 426 Running threads: 1 Questions: 11082 Reloads: 1 Open tables: 12

This can be somewhat perplexing if you only have 6 tables.

MySQL is multithreaded, so it may have many queries on the same table simultaneously. To minimize the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory and one extra file descriptor for the data file. The index file descriptor is shared between all threads.

10.10 Using symbolic links for databases and tables

You can move tables and databases from the database directory to other locations and replace them with symbolic links to the new locations. You might want to do this, for example, to move a database to a file system with more free space.

If MySQL notices that a table is a symbolically-linked, it will resolve the symlink and use the table it points to instead. This works on all systems that support the realpath() call (at least Linux and Solaris support realpath())! On systems that don't support realpath(), you should not access the table through the real path and through the symlink at the same time! If you do, the table will be inconsistent after any update.

MySQL doesn't support linking of databases by default. Things will work fine as long as you don't make a symbolic link between databases. Suppose you have a database db1 under the MySQL data directory, and then make a symlink db2 that points to db1:

shell> cd /path/to/datadir
shell> ln -s db1 db2

Now, for any table tbl_a in db1, there also appears to be a table tbl_a in db2. If one thread updates db1.tbl_a and another thread updates db2.tbl_a, there will be problems.

If you really need this, you must change the following code in `mysys/mf_format.c':

if (!lstat(to,&stat_buff))  /* Check if it's a symbolic link */
    if (S_ISLNK(stat_buff.st_mode) && realpath(to,buff))

Change the code to this:

if (realpath(to,buff))

10.11 How MySQL locks tables

All locking in MySQL is deadlock-free. This is managed by always requesting all needed locks at once at the beginning of a query and always locking the tables in the same order.

The locking method MySQL uses for WRITE locks works as follows:

The locking method MySQL uses for READ locks works as follows:

When a lock is released, the lock is made available to the threads in the write lock queue, then to the threads in the read lock queue.

This means that if you have many updates on a table, SELECT statements will wait until there are no more updates.

To work around this for the case where you want to do many INSERT and SELECT operations on a table, you can insert rows in a temporary table and update the real table with the records from the temporary table once in a while.

This can be done with the following code:

mysql> LOCK TABLES real_table WRITE, insert_table WRITE;
mysql> insert into real_table select * from insert_table;
mysql> delete from insert_table;
mysql> UNLOCK TABLES;

You can use the LOW_PRIORITY or HIGH_PRIORITY options with INSERT if you want to prioritize retrieval in some specific cases. See section 7.13 INSERT syntax.

You could also change the locking code in `mysys/thr_lock.c' to use a single queue. In this case, write locks and read locks would have the same priority, which might help some applications.

10.12 How to arrange a table to be as fast/small as possible

You can get better performance on a table and minimize storage space using the techniques listed below:

To check how fragmented your tables are, run isamchk -evi on the `.ISM' file. See section 13 Maintaining a MySQL installation.

10.13 Table locking issues

The table locking code in MySQL is deadlock free.

MySQL uses table locking (instead of row locking or column locking) to achieve a very high lock speed. For large tables, table locking is MUCH better than row locking, but there are of course some pitfalls.

Table locking enables many threads to read from a table at the same time, but if a thread wants to write to a table, it must first get exclusive access. During the update all others threads that want to access this particular table will wait until the update is ready.

As updates of databases normally are considered to be more important than SELECT, all statements that update a table have higher priority than statements that retrieve information from a table. This should ensure that updates are not 'starved' because one issues a lot of heavy queries against a specific table.

One main problem with this is the following:

Some possible solutions to this problem are:

10.14 Factors affecting the speed of INSERT statements

The time to insert a record consists of:

Where (number) is proportional time. This does not take into consideration the initial overhead to open tables (which is done once for each concurrently-running query).

The size of the table slows down the insertion of indexes by N log N (B-trees).

You can speed up insertions by locking your table and/or using multiple value lists with INSERT statements. Using multiple value lists can be up to 5 times faster than using separate inserts.

mysql> LOCK TABLES a WRITE;
mysql> INSERT INTO a VALUES (1,23),(2,34),(4,33);
mysql> INSERT INTO a VALUES (8,26),(6,29);
mysql> UNLOCK TABLES;

The main speed difference is that the index buffer is flushed to disk only once, after all INSERT statements have completed. Normally there would be as many index buffer flushes as there are different INSERT statements. Locking is not needed if you can insert all rows with a single statement.

Locking will also lower the total time of multi-connection tests, but the maximum wait time for some threads will go up (because they wait for locks). For example:

thread 1 does 1000 inserts
thread 2, 3, and 4 does 1 insert
thread 5 does 1000 inserts

If you don't use locking, 2, 3 and 4 will finish before 1 and 5. If you use locking, 2, 3 and 4 probably will not finish before 1 or 5, but the total time should be about 40% faster.

As INSERT, UPDATE and DELETE operations are very fast in MySQL, you will obtain better overall performance by adding locks around everything that does more than about 5 inserts or updates in a row. If you do very many inserts in a row, you could do a LOCK TABLES followed by a UNLOCK TABLES once in a while (about each 1000 rows) to allow other threads access to the table. This would still result in a nice performance gain.

Of course, LOAD DATA INFILE is much faster still.

If you are inserting a lot of rows from different clients, you can get higher speed by using the INSERT DELAYED statement. See section 7.13 INSERT syntax.

10.15 Factors affecting the speed of DELETE statements

The time to delete a record is exactly proportional to the number of indexes. To delete records more quickly, you can increase the size of the index cache. The default index cache is 1M; to get faster deletes, it should be increased by several factors (try 16M if you have enough memory).

10.16 How do I get MySQL to run at full speed?

Start by benchmarking your problem! You can take any program from the MySQL benchmark suite (normally found in the `sql-bench' directory) and modify it for your needs. By doing this, you can try different solutions to your problem and test which is really the fastest solution for you.

If your problem is with some explicit MySQL function, you can always time this in the MySQL client:

mysql> select benchmark(1000000,1+1);
+------------------------+
| benchmark(1000000,1+1) |
+------------------------+
|                      0 |
+------------------------+
1 row in set (0.32 sec)

The above shows that MySQL can execute 1,000,000 + expressions in 0.32 seconds on a simple PentiumII 400MHz.

All MySQL functions should be very optimized, but there may be some exceptions and the benchmark(loop_count,expression) is a great tool to find if this is a problem with your query.

10.17 What are the different row formats? Or, when should VARCHAR/CHAR be used?

MySQL dosen't have true SQL VARCHAR types.

Instead, MySQL has three different ways to store records and uses these to emulate VARCHAR.

If a table doesn't have any VARCHAR, BLOB or TEXT columns, a fixed row size is used. Otherwise a dynamic row size is used. CHAR and VARCHAR columns are treated identically from the application's point of view; both have trailing spaces removed when the columns are retrieved.

You can check the format used in a table with isamchk -d (-d means "describe the table").

MySQL has three different table formats: fixed-length, dynamic and compressed. These are compared below.

Fixed-length tables

Dynamic tables

Compressed tables

MySQL can support different index types, but the normal type is NISAM. This is a B-tree index and you can roughly calculate the size for the index file as (key_length+4)*0.67, summed over all keys. (This is for the worst case when all keys are inserted in sorted order.)

String indexes are space compressed. If the first index part is a string, it will also be prefix compressed. Space compression makes the index file smaller if the string column has a lot of trailing space or is a VARCHAR column that is not always used to the full length. Prefix compression helps if there are many strings with an identical prefix.

10.18 MySQL table types.

Table types are introduced in MySQL 3.23!

When you create a new table, you can tell MySQL which table type it should use for the table. MySQL will always create a .frm file to hold the table and column definitions. Depending on the table type the index and data will be stored in other files.

You can convert tables between different types with the ALTER TABLE statement. See section 7.7 ALTER TABLE syntax.

11 The MySQL benchmark suite

This should contain a technical description of the MySQL benchmark suite (and crash-me) but that description is not written yet. Currently, you should look at the code and results in the `bench' directory in the distribution (and of course on the web page at http://www.mysql.com/crash-me-choose.htmy).

It is meant to be a benchmark that will tell any user what things a given SQL implementation performs well or poorly at.

crash-me tries to determine what features a database supports and what its capabilities and limitations are by actually running queries. For example, it determines:

12 MySQL Utilites

12.1 Overview of the different MySQL programs

All MySQL clients that communicate with the server using the mysqlclient library use the following environment variables:

Name Description
MYSQL_UNIX_PORT The default socket; used for connections to localhost
MYSQL_TCP_PORT The default TCP/IP port
MYSQL_PWD The default password
MYSQL_DEBUG Debug-trace options when debugging
TMPDIR The directory where temporary tables/files are created

Use of MYSQL_PWD is insecure. See section 6.3 Connecting to the MySQL server.

The `mysql' client uses the file named in the MYSQL_HISTFILE environment variable to save the command line history. The default value for the history file is `$HOME/.mysql_history', where $HOME is the value of the HOME environment variable.

All MySQL programs take many different options. However, every MySQL program provides a --help option that you can use to get a full description of the program's different options. For example, try mysql --help.

You can override default options for all standard client programs with an option file. section 4.15.4 Option files.

The list below briefly describes the MySQL programs:

isamchk
Utility to describe, check, optimize and repair MySQL tables. Because isamchk has many functions, it is described in its own chapter. See section 13 Maintaining a MySQL installation.
make_binary_release
Makes a binary release of a compiled MySQL. This could be sent by FTP to `/pub/mysql/Incoming' on ftp.tcx.se for the convenience of other MySQL users.
msql2mysql
A shell script that converts mSQL programs to MySQL. It doesn't handle all cases, but it gives a good start when converting.
mysql
mysql is a simple SQL shell (with GNU readline capabilities). It supports interactive and non-interactive use. When used interactively, query results are presented in an ASCII-table format. When used non-interactively (e.g., as a filter), the result is presented in tab-separated format. (The output format can be changed using command-line options.) You can run scripts simply like this:
shell> mysql database < script.sql > output.tab
If you have problems due to insufficient memory in the client, use the --quick option! This forces mysql to use mysql_use_result() rather than mysql_store_result() to retrieve the result set.
mysqlaccess
A script that checks the access privileges for a host, user and database combination.
mysqladmin
Utility for performing administrative operations, such as creating or dropping databases, reloading the grant tables, flushing tables to disk and reopening log files. mysqladmin can also be used to retrieve version, process and status information from the server. See section 12.2 Administering a MySQL server.
mysqlbug
The MySQL bug report script. This script should always be used when filing a bug report to the MySQL list.
mysqld
The SQL daemon. This should always be running.
mysqldump
Dumps a MySQL database into a file as SQL statements or as tab-separated text files. Enhanced freeware originally by Igor Romanenko. See section 12.3 Dumping the structure and data from MySQL databases and tables.
mysqlimport
Imports text files into their respective tables using LOAD DATA INFILE. See section 12.4 Importing data from text files.
mysqlshow
Displays information about databases, tables, columns and indexes.
mysql_install_db
Creates the MySQL grant tables with default privileges. This is usually executed only once, when first installing MySQL on a system.
replace
A utility program that is used by msql2mysql, but that has more general applicability as well. replace changes strings in place in files or on the standard input. Uses a finite state machine to match longer strings first. Can be used to swap strings. For example, this command swaps a and b in the given files:
shell> replace a b b a -- file1 file2 ...
safe_mysqld
A script that starts the mysqld daemon with some safety features, such as restarting the server when an error occurs and logging runtime information to a log file.

12.2 Administering a MySQL server

Utility for performing administrative operations. The syntax is:

shell> mysqladmin [OPTIONS] command [command-option] command ...

You can get a list of the options your version of mysqladmin supports by executing mysqladmin --help.

The current mysqladmin supports the following commands:

create databasename Create a new database.
drop databasename Delete a database and all its tables.
extended-status Gives an extended status message from the server.
flush-hosts Flush all cached hosts.
flush-logs Flush all logs.
flush-tables Flush all tables.
flush-privileges Reload grant tables (same as reload)
kill id,id,... Kill mysql threads.
password new-password Change old password to new-password
ping Check if mysqld is alive
processlist Show list of active threads in server
reload Reload grant tables
refresh Flush all tables and close and open logfiles
shutdown Take server down
status Gives a short status message from the server
variables Prints variables available
version Get version info from server

All commands can be shortened to their unique prefix. For example:

shell> mysqladmin proc stat
+----+-------+-----------+----+-------------+------+-------+------+
| Id | User  | Host      | db | Command     | Time | State | Info |
+----+-------+-----------+----+-------------+------+-------+------+
| 6  | monty | localhost |    | Processlist | 0    |       |      |
+----+-------+-----------+----+-------------+------+-------+------+
Uptime: 10077  Threads: 1  Questions: 9  Slow queries: 0  Opens: 6  Flush tables: 1  Open tables: 2  Memory in use: 1092K  Max memory used: 1116K

The mysqladmin status command result has the following columns:

Uptime Number of seconds the MySQL server have been up
Threads Number of active threads (clients)
Questions Number of questions from clients since mysqld was started
Slow queries Queries that has taken more than long_query_time seconds
Opens How many tables mysqld has opened.
Flush tables Number of flush ..., refresh and reload commands.
Open tables Number of tables that are open now
Memory in use Memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug)
Max memory used Maximum memory allocated directly by the mysqld code (only available when MySQL is compiled with --with-debug)

12.3 Dumping the structure and data from MySQL databases and tables

Utility to dump a database or a collection of database for backup or for transferring the data to another SQL server. The dump will contain SQL statements to create the table and/or populate the table.

shell> mysqldump [OPTIONS] database [tables]

If you don't give any tables, the whole database will be dumped.

You can get a list of the options your version of mysqldump supports by executing mysqldump --help.

Note that if you run mysqldump without --quick or --opt, mysqldump will load the whole result set into memory before dumping the result. This will probably be a problem if you are dumping a big database.

mysqldump supports the following options:

--add-locks
Add LOCK TABLES before and UNLOCK TABLE after each table dump. (To get faster inserts into MySQL).
--add-drop-table
Add a drop table before each create statement.
--allow-keywords
Allow creation of column names that are keywords. This works by prefixing each column name with the table name.
-c, --complete-insert
Use complete insert statements (with column names).
-C, --compress
Compress all information between the client and the server if both support compression.
--delayed
Insert rows with the INSERT DELAYED command.
-e, --extended-insert
Use the new multiline INSERT syntax. (Gives more compact and faster inserts statements)
-#, --debug[=option_string]
Trace usage of the program (for debugging).
--help
Display a help message and exit.
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--fields-terminated-by=...
These options are used with the -T option and have the same meaning as the corresponding clauses for LOAD DATA INFILE. See section 7.15 LOAD DATA INFILE syntax.
-F, --flush-logs
Flush logs file in the MySQL server before starting the dump.
-f, --force,
Continue even if we get an SQL error during a table dump.
-h, --host=..
Dump data from the MySQL server on the named host. The default host is localhost.
-l, --lock-tables.
Lock all tables for starting the dump.
-t, --no-create-info
Don't write table creation info (The CREATE TABLE statment)
-d, --no-data
Don't write any row information for the table. This is very useful if you just want to get a dump of the structure for a table!
--opt
Same as --quick --add-drop-table --add-locks --extended-insert --use-locks. Should give you the fastest possible dump for reading into a MySQL server.
-pyour_pass, --password[=your_pass]
The password to use when connecting to the server. If you specify no `=your_pass' part, mysqldump solicits the password from the terminal.
-P port_num, --port=port_num
The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than localhost, for which Unix sockets are used.)
-q, --quick
Don't buffer query, dump directly to stdout; Uses mysql_use_result() to do this.
-S /path/to/socket, --socket=/path/to/socket
The socket file to use when connecting to localhost (which is the default host).
-T, --tab=path-to-some-directory
Creates a table_name.sql file, that conntains the SQL CREATE commands, and a table_name.txt file, that contains the data, for each give table. NOTE: This only works if mysqldump is run on the same machine as the mysqld daemon. The format of the .txt file is made according to the --fields-xxx and --lines--xxx options.
-u user_name, --user=user_name
The MySQL user name to use when connecting to the server. The default value is your Unix login name.
-O var=option, --set-variable var=option
Set the value of a variable. The possible variables are listed below.
-v, --verbose
Verbose mode. Print out more information what the program does.
-V, --version
Print version information and exit.
-w, --where='where-condition'
Dump only selected records; Note that QUOTES are mandatory!
"--where=user='jimf'" "-wuserid>1" "-wuserid<1"

The most normal use of mysqldump is probably for making a backup of whole database:

mysqldump --opt database > backup-file.sql

But it's also very useful to populate another MySQL server with information from a databas:

mysqldump --opt database | mysql --host=remote-host -C database

12.4 Importing data from text files

mysqlimport provides a command line interface to the LOAD DATA INFILE SQL statement. Most options to mysqlimport correspond directly to the same options to LOAD DATA INFILE. See section 7.15 LOAD DATA INFILE syntax.

mysqlimport is invoked like this:

shell> mysqlimport [options] filename ...

For each text file named on the command line, mysqlimport strips any extension from the filename and uses the result to determine which table to import the file's contents into. For example, files named `patient.txt', `patient.text' and `patient' would all be imported into a table named patient.

mysqlimport supports the following options:

-C, --compress
Compress all information between the client and the server if both support compression.
-#, --debug[=option_string]
Trace usage of the program (for debugging).
-d, --delete
Empty the table before importing the text file.
--fields-terminated-by=...
--fields-enclosed-by=...
--fields-optionally-enclosed-by=...
--fields-escaped-by=...
--fields-terminated-by=...
These options have the same meaning as the corresponding clauses for LOAD DATA INFILE. See section 7.15 LOAD DATA INFILE syntax.
-f, --force
Ignore errors. For example, if a table for a text file doesn't exist, continue processing any remaining files. Without --force, mysqlimport exits if a table doesn't exist.
--help
Display a help message and exit.
-h host_name, --host=host_name
Import data to the MySQL server on the named host. The default host is localhost.
-i, --ignore
See the description for the --replace option.
-l, --lock-tables
Lock ALL tables for writing before processing any text files. This ensures that all tables are synchronized on the server.
-L, --local
Read input files from the client. By default, text files are assumed to be on the server if you connect to localhost (which is the default host).
-pyour_pass, --password[=your_pass]
The password to use when connecting to the server. If you specify no `=your_pass' part, mysqlimport solicits the password from the terminal.
-P port_num, --port=port_num
The TCP/IP port number to use for connecting to a host. (This is used for connections to hosts other than localhost, for which Unix sockets are used.)
-r, --replace
The --replace and --ignore options control handling of input records that duplicate existing records on unique key values. If you specify --replace, new rows replace existing rows that have the same unique key value. If you specify --ignore, input rows that duplicate an existing row on a unique key value are skipped. If you don't specify either option, an error occurs when a duplicate key value is found, and the rest of the text file is ignored.
-s, --silent
Silent mode. Write output only when errors occur.
-S /path/to/socket, --socket=/path/to/socket
The socket file to use when connecting to localhost (which is the default host).
-u user_name, --user=user_name
The MySQL user name to use when connecting to the server. The default value is your Unix login name.
-v, --verbose
Verbose mode. Print out more information what the program does.
-V, --version
Print version information and exit.

12.5 The MySQL compressed read-only table generator

pack_isam is an extra utility that you get when you order more than 10 licenses or extended support. Since pack_isam is distributed only in binary form, pack_isam is available only on some platforms.

Of course, all future updates to pack_isam are included in the price. pack_isam may at some time be included as standard when we get some kind of turnover for MySQL.

pack_isam works by compressing each column in the table separately. The information needed to decompress columns is read into memory when the table is opened. This results in much better performance when accessing individual records, since you only have to uncompress exactly one record, not a much larger disk block like when using Stacker on MS-DOS. Usually, pack_isam packs the data file 40%-70%.

MySQL uses memory mapping (mmap()) on compressed tables and falls back to normal read/write file usage if mmap() doesn't work.

There are currently two limitations with pack_isam:

Fixing these limitations is on our TODO list but with low priority.

pack_isam is invoked like this:

shell> pack_isam [options] filename ...

Each filename should be the name of an index (`.ISM') file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the `.ISM' extension.

pack_isam supports the following options:

-b, --backup
Make a backup of the table as tbl_name.OLD.
-#, --debug=debug_options
Output debug log. The debug_options string often is 'd:t:o,filename'.
-f, --force
Force packing of the table even if it becomes bigger or if the temporary file exists. (pack_isam creates a temporary file named `tbl_name.TMD' while it compresses the table. If you kill pack_isam, the `.TMD' file may not be deleted. Normally, pack_isam exits with an error if it finds that `tbl_name.TMD' exists. With --force, pack_isam packs the table anyway.
-?, --help
Display a help message and exit.
-j big_tbl_name, --join=big_tbl_name
Join all tables named on the command line into a single table big_tbl_name. All tables that are to be combined MUST be identical (same column names and types, same indexes, etc.)
-p #, --packlength=#
Specify the record length storage size, in bytes. The value should be 1, 2 or 3. (pack_isam stores all rows with length pointers of 1, 2 or 3 bytes. In most normal cases, pack_isam can determine the right length value before it begins packing the file, but it may notice during the packing process that it could have used a shorter length. In this case, pack_isam will print a note that the next time you pack the same file, you could use a shorter record length.)
-s, --silent
Silent mode. Write output only when errors occur.
-t, --test
Don't pack table, only test packing it.
-T dir_name, --tmp_dir=dir_name
Use the named directory as the location in which to write the temporary table.
-v, --verbose
Verbose mode. Write info about progress and packing result.
-V, --version
Display version information and exit.
-w, --wait
Wait and retry if table is in use. If the mysqld server was invoked with the --skip-locking option, it is not a good idea to invoke pack_isam if the table might be updated during the packing process.

The sequence of commands shown below illustrates a typical table compression session:

shell> ls -l station.*
-rw-rw-r--   1 monty    my         994128 Apr 17 19:00 station.ISD
-rw-rw-r--   1 monty    my          53248 Apr 17 19:00 station.ISM
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> isamchk -dvv station

ISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-02-02  3:06:43
Data records:              1192  Deleted blocks:              0
Datafile: Parts:           1192  Deleted data:                0
Datafile pointer (bytes):     2  Keyfile pointer (bytes):     2
Max datafile length:   54657023  Max keyfile length:   33554431
Recordlength:               834
Record format: Fixed length

table description:
Key Start Len Index   Type                       Root  Blocksize    Rec/key
1   2     4   unique  unsigned long              1024       1024          1
2   32    30  multip. text                      10240       1024          1

Field Start Length Type
1     1     1
2     2     4
3     6     4
4     10    1
5     11    20
6     31    1
7     32    30
8     62    35
9     97    35
10    132   35
11    167   4
12    171   16
13    187   35
14    222   4
15    226   16
16    242   20
17    262   20
18    282   20
19    302   30
20    332   4
21    336   4
22    340   1
23    341   8
24    349   8
25    357   8
26    365   2
27    367   2
28    369   4
29    373   4
30    377   1
31    378   2
32    380   8
33    388   4
34    392   4
35    396   4
36    400   4
37    404   1
38    405   4
39    409   4
40    413   4
41    417   4
42    421   4
43    425   4
44    429   20
45    449   30
46    479   1
47    480   1
48    481   79
49    560   79
50    639   79
51    718   79
52    797   8
53    805   1
54    806   1
55    807   20
56    827   4
57    831   4

shell> pack_isam station.ISM
Compressing station.ISM: (1192 records)
- Calculating statistics

normal:     20  empty-space:      16  empty-zero:        12  empty-fill:  11
pre-space:   0  end-space:        12  table-lookups:      5  zero:         7
Original trees:  57  After join: 17
- Compressing file
87.14%

shell> ls -l station.*
-rw-rw-r--   1 monty    my         127874 Apr 17 19:00 station.ISD
-rw-rw-r--   1 monty    my          55296 Apr 17 19:04 station.ISM
-rw-rw-r--   1 monty    my           5767 Apr 17 19:00 station.frm

shell> isamchk -dvv station

ISAM file:     station
Isam-version:  2
Creation time: 1996-03-13 10:08:58
Recover time:  1997-04-17 19:04:26
Data records:              1192  Deleted blocks:              0
Datafile: Parts:           1192  Deleted data:                0
Datafilepointer (bytes):      3  Keyfile pointer (bytes):     1
Max datafile length:   16777215  Max keyfile length:     131071
Recordlength:               834
Record format: Compressed

table description:
Key Start Len Index   Type                       Root  Blocksize    Rec/key
1   2     4   unique  unsigned long             10240       1024          1
2   32    30  multip. text                      54272       1024          1

Field Start Length Type                         Huff tree  Bits
1     1     1      constant                             1     0
2     2     4      zerofill(1)                          2     9
3     6     4      no zeros, zerofill(1)                2     9
4     10    1                                           3     9
5     11    20     table-lookup                         4     0
6     31    1                                           3     9
7     32    30     no endspace, not_always              5     9
8     62    35     no endspace, not_always, no empty    6     9
9     97    35     no empty                             7     9
10    132   35     no endspace, not_always, no empty    6     9
11    167   4      zerofill(1)                          2     9
12    171   16     no endspace, not_always, no empty    5     9
13    187   35     no endspace, not_always, no empty    6     9
14    222   4      zerofill(1)                          2     9
15    226   16     no endspace, not_always, no empty    5     9
16    242   20     no endspace, not_always              8     9
17    262   20     no endspace, no empty                8     9
18    282   20     no endspace, no empty                5     9
19    302   30     no endspace, no empty                6     9
20    332   4      always zero                          2     9
21    336   4      always zero                          2     9
22    340   1                                           3     9
23    341   8      table-lookup                         9     0
24    349   8      table-lookup                        10     0
25    357   8      always zero                          2     9
26    365   2                                           2     9
27    367   2      no zeros, zerofill(1)                2     9
28    369   4      no zeros, zerofill(1)                2     9
29    373   4      table-lookup                        11     0
30    377   1                                           3     9
31    378   2      no zeros, zerofill(1)                2     9
32    380   8      no zeros                             2     9
33    388   4      always zero                          2     9
34    392   4      table-lookup                        12     0
35    396   4      no zeros, zerofill(1)               13     9
36    400   4      no zeros, zerofill(1)                2     9
37    404   1                                           2     9
38    405   4      no zeros                             2     9
39    409   4      always zero                          2     9
40    413   4      no zeros                             2     9
41    417   4      always zero                          2     9
42    421   4      no zeros                             2     9
43    425   4      always zero                          2     9
44    429   20     no empty                             3     9
45    449   30     no empty                             3     9
46    479   1                                          14     4
47    480   1                                          14     4
48    481   79     no endspace, no empty               15     9
49    560   79     no empty                             2     9
50    639   79     no empty                             2     9
51    718   79     no endspace                         16     9
52    797   8      no empty                             2     9
53    805   1                                          17     1
54    806   1                                           3     9
55    807   20     no empty                             3     9
56    827   4      no zeros, zerofill(2)                2     9
57    831   4      no zeros, zerofill(1)                2     9

The information printed by pack_isam is described below:

normal
The number of columns for which no extra packing is used.
empty-space
The number of columns containing values that are only spaces; these will occupy 1 bit.
empty-zero
The number of columns containing values that are only binary 0's; these will occupy 1 bit.
empty-fill
The number of integer columns that don't occupy the full byte range of their type; these are changed to a smaller type (for example, an INTEGER column may be changed to MEDIUMINT).
pre-space
The number of decimal columns that are stored with leading space. In this case, each value will contain a count for the number of leading spaces.
end-space
The number of columns that have a lot of trailing space. In this case, each value will contain a count for the number of trailing spaces.
table-lookup
The column had only a small number of different values, and that were converted to an ENUM before Huffman compression.
zero
The number of columns for which all values are zero.
Original trees
The initial number of Huffman trees.
After join
The number of distinct Huffman trees left after joining trees to save some header space.

After a table has been compressed, isamchk -dvv prints additional information about each field:

Type
The field type may contain the following descriptors:
constant
All rows have the same value.
no endspace
Don't store endspace.
no endspace, not_always
Don't store endspace and don't do end space compression for all values.
no endspace, no empty
Don't store endspace. Don't store empty values.
table-lookup
The column was converted to an ENUM.
zerofill(n)
The most significant n bytes in the value are always 0 and are not stored.
no zeros
Don't store zeros.
always zero
0 values are stored in 1 bit.
Huff tree
The Huffman tree associated with the field
Bits
The number of bits used in the Huffman tree.

13 Maintaining a MySQL installation

13.1 Using isamchk for table maintenance and crash recovery

To check/repair ISAM tables (.ISM and .ISD) you should use the isamchk utility. To check/repair MyISAM tables (.MYI and .MYD) you should use the myisamchk utility. See section 10.18 MySQL table types..

In the following text we will talk about isamchk but everything also applies to myisamchk.

You can use the isamchk utility to get information about your database tables, check and repair them or optimize them. The following sections describe how to invoke isamchk (including a description of its options), how to set up a table maintenance schedule, and how to use isamchk to perform its various functions.

If you run mysqld with --skip-locking (which is the default on some systems, like Linux), you can't reliably use isamchk to check a table when mysqld is using the same table. If you can be sure that no one is accessing the tables through mysqld while you run isamchk, you only have to do mysqladmin flush-tables before you start checking the tables. If you can't guarantee the above, then you must take down mysqld while you check the tables. If you run isamchk while mysqld is updating the tables, you may get a warning that a table is corrupt even if it isn't.

If you are not using --skip-locking, you can use isamchk to check tables at any time. While you do this, all clients that try to update the table will wait until isamchk is ready before continuing.

If you use isamchk to repair or optimize tables, you MUST always ensure that the mysqld server is not using the table (this also applies if you are using --skip-locking). If you don't take down mysqld you should at least do a mysqladmin flush-tables before you run isamchk.

You can in most cases also use the command OPTIMIZE TABLES to optimize and repair tables, but this is not as fast or reliable (in case of real fatal errors) as isamchk. On the other hand, OPTIMIZE TABLE is easier to use and you don't have to worry about flushing tables. See section 7.8 OPTIMIZE TABLE syntax.

13.1.1 isamchk invocation syntax

isamchk is invoked like this:

shell> isamchk [options] tbl_name

The options specify what you want isamchk to do. They are described below. (You can also get a list of options by invoking isamchk --help.) With no options, isamchk simply checks your table. To get more information or to tell isamchk to take corrective action, specify options as described below and in the following sections.

tbl_name is the database table you want to check. If you run isamchk somewhere other than in the database directory, you must specify the path to the file, since isamchk has no idea where your database is located. Actually, isamchk doesn't care whether or not the files you are working on are located in a database directory; you can copy the files that correspond to a database table into another location and perform recovery operations on them there.

You can name several tables on the isamchk command line if you wish. You can also specify a name as an index file name (with the `.ISM' suffix), which allows you to specify all tables in a directory by using the pattern `*.ISM'. For example, if you are in a database directory, you can check all the tables in the directory like this:

shell> isamchk *.ISM

If you are not in the database directory, you can check all the tables there by specifying the path to the directory:

shell> isamchk /path/to/database_dir/*.ISM

You can even check all tables in all databases by specifying a wildcard with the path to the MySQL data directory:

shell> isamchk /path/to/datadir/*/*.ISM

isamchk supports the following options:

-a, --analyze
Analyze the distribution of keys. This improves join performance by enabling the join optimizer to better choose in which order it should join the tables and which keys it should how use.
-#, --debug=debug_options
Output debug log. The debug_options string often is 'd:t:o,filename'.
-d, --description
Prints some information about the table.
-e, --extend-check
Check the table VERY thoroughly. This is necessary only in extreme cases. Normally, isamchk should find all errors even without this option.
-f, --force
Overwrite old temporary files. If you use -f when checking tables (running isamchk without -r), isamchk will automatically restart with -r on any table for which an error occurs during checking.
--help
Display a help message and exit.
-i, --information
Print informational statistics about the table that is checked.
-k #, --keys-used=#
Used with -r. Tell the NISAM table handler to update only the first # indexes. Higher-numbered indexes are deactivated. This can be used to get faster inserts! Deactivated indexes can be reactivated by using isamchk -r.
-l, --no-symlinks
Do not follow symbolic links when repairing. Normally isamchk repairs the table a symlink points at.
-q, --quick
Used with -r to get a faster repair. Normally, the original data file isn't touched; you can specify a second -q to force the original data file to be used.
-r, --recover
Recovery mode. Can fix almost anything except unique keys that aren't unique.
-o, --safe-recover
Recovery mode. Uses an old recovery method; this is slower than -r, but can handle a couple of cases that -r cannot handle.
-O var=option, --set-variable var=option
Set the value of a variable. The possible variables are listed below.
-s, --silent
Silent mode. Write output only when errors occur. You can use -s twice (-ss) to make isamchk very silent.
-S, --sort-index
Sort the index tree blocks in high-low order. This will optimize seeks and will make table scanning by key faster.
-R index_num, --sort-records=index_num
Sorts records according to an index. This makes your data much more localized and may speed up ranged SELECT and ORDER BY operations on this index. (It may be VERY slow to do a sort the first time!) To find out a table's index numbers, use SHOW INDEX, which shows a table's indexes in the same order that isamchk sees them. Indexes are numbered beginning with 1.
-u, --unpack
Unpack a table that was packed with pack_isam.
-v, --verbose
Verbose mode. Print more information. This can be used with -d and -e. Use -v multiple times (-vv, -vvv) for more verbosity!
-V, --version
Print the isamchk version and exit.
-w, --wait
Wait if the table is locked.

Possible variables for the --set-variable (-O) option are:

key_buffer_size       current value: 16776192
read_buffer_size      current value: 262136
write_buffer_size     current value: 262136
sort_buffer_size      current value: 2097144
sort_key_blocks       current value: 16
decode_bits           current value: 9

13.1.2 isamchk memory usage

Memory allocation is important when you run isamchk. isamchk uses no more memory than you specify with the -O options. If you are going to use isamchk on very large files, you should first decide how much memory you want it to use. The default is to use only about 3M to fix things. By using larger values, you can get isamchk to operate faster. For example, if you have more than 32M RAM, you could use options such as these (in addition to any other options you might specify):

shell> isamchk -O sort=16M -O key=16M -O read=1M -O write=1M ...

Using -O sort=16M should probably be enough for most cases.

Be aware that isamchk uses temporary files in TMPDIR. If TMPDIR points to a memory file system, you may easily get out of memory errors. If this happens, set TMPDIR to point at some directory with more space and restart isamchk

13.2 Setting up a table maintenance regimen

It is a good idea to perform table checks on a regular basis rather than waiting for problems to occur. For maintenance purposes, you can use isamchk -s to check tables. The -s option causes isamchk to run in silent mode, printing messages only when errors occur.

It's a good idea to check tables when the server starts up. For example, whenever the machine has done a reboot in the middle of an update, you usually need to check all the tables that could have been affected. (This is an "expected crashed table".) You could add a test to safe_mysqld that runs isamchk to check all tables that have been modified during the last 24 hours if there is an old `.pid' (process ID) file left after a reboot. (The `.pid' file is created by mysqld when it starts up and removed when it terminates normally. The presence of a `.pid' file at system startup time indicates that mysqld terminated abnormally.)

An even better test would be to check any table whose last-modified time is more recent than that of the `.pid' file.

You should also check your tables regularly during normal system operation. At TcX, we run a cron job to check all our important tables once a week, using a line like this in a `crontab' file:

35 0 * * 0 /path/to/isamchk -s /path/to/datadir/*/*.ISM

This prints out information about crashed tables so we can examine and repair them when needed.

As we haven't had any unexpectedly crashed tables (tables that become corrupted for reasons other than hardware trouble) for a couple of years now (this is really true), once a week is more than enough for us.

We recommend that to start with, you execute isamchk -s each night on all tables that have been updated during the last 24 hours, until you come to trust MySQL as much as we do.

13.3 Getting information about a table

To get a description of a table or statistics about it, use the commands shown below. We explain some of the information in more detail later.

isamchk -d tbl_name
Runs isamchk in "describe mode" to produce a description of your table. If you start the MySQL server using the --skip-locking option, isamchk may report an error for a table that is updated while it runs. However, since isamchk doesn't change the table in describe mode, there isn't any risk of destroying data.
isamchk -d -v tbl_name
To produce more information about what isamchk is doing, add -v to tell it to run in verbose mode.
isamchk -eis tbl_name
Shows only the most important information from a table. It is slow since it must read the whole table.
isamchk -eiv tbl_name
This is like -eis, but tells you what is being done.

Example of isamchk -d output:

ISAM file:     company.ISM
Data records:           1403698  Deleted blocks:         0
Recordlength:               226
Record format: Fixed length

table description:
Key Start Len Index   Type
1   2     8   unique  double
2   15    10  multip. text packed stripped
3   219   8   multip. double
4   63    10  multip. text packed stripped
5   167   2   multip. unsigned short
6   177   4   multip. unsigned long
7   155   4   multip. text
8   138   4   multip. unsigned long
9   177   4   multip. unsigned long
    193   1           text

Example of isamchk -d -v output:

ISAM file:     company.ISM
Isam-version:  2
Creation time: 1996-08-28 11:44:22
Recover time:  1997-01-12 18:35:29
Data records:           1403698  Deleted blocks:              0
Datafile: Parts:        1403698  Deleted data:                0
Datafilepointer (bytes):      3  Keyfile pointer (bytes):     3
Max datafile length: 3791650815  Max keyfile length: 4294967294
Recordlength:               226
Record format: Fixed length

table description:
Key Start Len Index   Type                      Root Blocksize Rec/key
1   2     8   unique  double                15845376      1024       1
2   15    10  multip. text packed stripped  25062400      1024       2
3   219   8   multip. double                40907776      1024      73
4   63    10  multip. text packed stripped  48097280      1024       5
5   167   2   multip. unsigned short        55200768      1024    4840
6   177   4   multip. unsigned long         65145856      1024    1346
7   155   4   multip. text                  75090944      1024    4995
8   138   4   multip. unsigned long         85036032      1024      87
9   177   4   multip. unsigned long         96481280      1024     178
    193   1           text

Example of isamchk -eis output:

Checking ISAM file: company.ISM
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:  98%  Packed:   17%

Records:          1403698    M.recordlength:     226   Packed:             0%
Recordspace used:     100%   Empty space:          0%  Blocks/Record:   1.00
Recordblocks:     1403698    Deleteblocks:         0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1626.51, System time 232.36
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 627, Swaps 0
Blocks in 0 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 639, Involuntary context switches 28966

Example of isamchk -eiv output:

Checking ISAM file: company.ISM
Data records: 1403698   Deleted blocks:       0
- check file-size
- check delete-chain
index  1:
index  2:
index  3:
index  4:
index  5:
index  6:
index  7:
index  8:
index  9:
No recordlinks
- check index reference
- check data record references index: 1
Key:  1:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 2
Key:  2:  Keyblocks used:  98%  Packed:   50%  Max levels:  4
- check data record references index: 3
Key:  3:  Keyblocks used:  97%  Packed:    0%  Max levels:  4
- check data record references index: 4
Key:  4:  Keyblocks used:  99%  Packed:   60%  Max levels:  3
- check data record references index: 5
Key:  5:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 6
Key:  6:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 7
Key:  7:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 8
Key:  8:  Keyblocks used:  99%  Packed:    0%  Max levels:  3
- check data record references index: 9
Key:  9:  Keyblocks used:  98%  Packed:    0%  Max levels:  4
Total:    Keyblocks used:   9%  Packed:   17%

- check records and index references
[LOTS OF ROW NUMBERS DELETED]

Records:          1403698    M.recordlength:     226   Packed:             0%
Recordspace used:     100%   Empty space:          0%  Blocks/Record:   1.00
Recordblocks:     1403698    Deleteblocks:         0
Recorddata:     317235748    Deleted data:         0
Lost space:             0    Linkdata:             0

User time 1639.63, System time 251.61
Maximum resident set size 0, Integral resident set size 0
Non physical pagefaults 0, Physical pagefaults 10580, Swaps 0
Blocks in 4 out 0, Messages in 0 out 0, Signals 0
Voluntary context switches 10604, Involuntary context switches 122798

Here are the sizes of the data and index files for the table used in the preceding examples:

-rw-rw-r--   1 monty    tcx     317235748 Jan 12 17:30 company.ISD
-rw-rw-r--   1 davida   tcx      96482304 Jan 12 18:35 company.ISM

Explanations for the types of information isamchk produces are given below. The "keyfile" is the index file. "Record" and "row" are synonymous.

ISAM file
Name of the ISAM (index) file.
Isam-version
Version of ISAM format. Currently always 2.
Creation time
When the data file was created.
Recover time
When the index/data file was last reconstructed.
Data records
How many records are in the table.
Deleted blocks
How many deleted blocks still have reserved space. You can optimize your table to minimize this space. See section 13.4.3 Table optimization.
Datafile: Parts
For dynamic record format, this indicates how many data blocks there are. For an optimized table without fragmented records, this is the same as Data records.
Deleted data
How many bytes of non-reclaimed deleted data there are. You can optimize your table to minimize this space. See section 13.4.3 Table optimization.
Datafile pointer
The size of the data file pointer, in bytes. It is usually 2, 3, 4 or 5 bytes. Most tables manage with 2 bytes, but this cannot be controlled from MySQL yet. For fixed tables, this is a record address. For dynamic tables, this is a byte address.
Keyfile pointer
The size of the index file pointer, in bytes. It is usually 1, 2 or 3 bytes. Most tables manage with 2 bytes, but this is calculated automatically by MySQL. It is always a block address.
Max datafile length
How long the table's data file (.ISD file) can become, in bytes.
Max keyfile length
How long the table's key file (.ISM file) can become, in bytes.
Recordlength
How much space each record takes, in bytes.
Record format
The format used to store table rows. The examples shown above use Fixed length. Other possible values are Compressed and Packed.
table description
A list of all keys in the table. For each key, some low-level information is presented:
Key
This key's number.
Start
Where in the record this index part starts.
Len
How long this index part is. For packed numbers, this should always be the full length of the column. For strings, it may be shorter than the full length of the indexed column, because you can index a prefix of a string column.
Index
unique or multip. (multiple). Indicates whether or not one value can exist multiple times in this index.
Type
What data-type this index part has. This is an NISAM data-type with the options packed, stripped or empty.
Root
Address of the root index block.
Blocksize
The size of each index block. By default this is 1024, but the value may be changed at compile time.
Rec/key
This is a statistical value used by the optimizer. It tells how many records there are per value for this key. A unique key always has a value of 1. This may be updated after a table is loaded (or greatly changed) with isamchk -a. If this is not updated at all, a default value of 30 is given.
In the first example above, the 9th key is a multi-part key with two parts.
Keyblocks used
What percentage of the keyblocks are used. Since the table used in the examples had just been reorganized with isamchk, the values are very high (very near the theoretical maximum).
Packed
MySQL tries to pack keys with a common suffix. This can only be used for CHAR/VARCHAR/DECIMAL keys. For long strings like names, this can significantly reduce the space used. In the third example above, the 4th key is 10 characters long and a 60% reduction in space is achieved.
Max levels
How deep the B-tree for this key is. Large tables with long keys get high values.
Records
How many rows are in the table.
M.recordlength
The average record length. For tables with fixed-length records, this is the exact record length.
Packed
MySQL strips spaces from the end of strings. The Packed value indicates the percentage savings achieved by doing this.
Recordspace used
What percentage of the data file is used.
Empty space
What percentage of the data file is unused.
Blocks/Record
Average number of blocks per record (i.e., how many links a fragmented record is composed of). This is always 1 for fixed-format tables. This value should stay as close to 1.0 as possible. If it gets too big, you can reorganize the table with isamchk. See section 13.4.3 Table optimization.
Recordblocks
How many blocks (links) are used. For fixed format, this is the same as the number of records.
Deleteblocks
How many blocks (links) are deleted.
Recorddata
How many bytes in the data file are used.
Deleted data
How many bytes in the data file are deleted (unused).
Lost space
If a record is updated to a shorter length, some space is lost. This is the sum of all such losses, in bytes.
Linkdata
When the dynamic table format is used, record fragments are linked with pointers (4 to 7 bytes each). Linkdata is the sum of the amount of storage used by all such pointers.

If a table has been compressed with pack_isam, isamchk -d prints additional information about each table column. See section 12.5 The MySQL compressed read-only table generator, for an example of this information and a description of what it means.

13.4 Using isamchk for crash recovery

The file format that MySQL uses to store data has been extensively tested, but there are always external circumstances that may cause database tables to become corrupted:

This chapter describes how to check for and deal with data corruption in MySQL databases. If your tables get corrupted a lot you should try to find the reason for this! See section G.1 Debugging a MySQL server.

When performing crash recovery, it is important to understand that each table tbl_name in a database corresponds to three files in the database directory:

File Purpose
`tbl_name.frm' Table definition (form) file
`tbl_name.ISD' Data file
`tbl_name.ISM' Index file

Each of these three file types is subject to corruption in various ways, but problems occur most often in data files and index files.

isamchk works by creating a copy of the `.ISD' (data) file row by row. It ends the repair stage by removing the old `.ISD' file and renaming the new file to the original file name. If you use --quick, isamchk does not create a temporary `.ISD' file, but instead assumes that the `.ISD' file is correct and only generates a new index file without touching the `.ISD' file. This is safe, because isamchk automatically detects if the `.ISD' file is corrupt and aborts the repair in this case. You can also give two --quick options to isamchk. In this case, isamchk does not abort on some errors (like duplicate key) but instead tries to resolve them by modifying the `.ISD' file. Normally the use of two --quick options is useful only if you have too little free disk space to perform a normal repair. In this case you should at least make a backup before running isamchk.

13.4.1 How to check tables for errors

To check a table, use the following commands:

isamchk tbl_name
This finds 99.99% of all errors. What it can't find is corruption that involves ONLY the data file (which is very unusual). If you want to check a table, you should normally run isamchk without options or with either the -s or --silent option.
isamchk -e tbl_name
This does a complete and thorough check of all data (-e means "extended check"). It does a check-read of every key for each row to verify that they indeed point to the correct row. This may take a LONG time on a big table with many keys. isamchk will normally stop after the first error it finds. If you want to obtain more information, you can add the --verbose (-v) option. This causes isamchk to keep going, up through a maximum of 20 errors. In normal usage, a simple isamchk (with no arguments other than the table name) is sufficient.
isamchk -e -i tbl_name
Like the previous command, but the -i option tells isamchk to print some informational statistics, too.

13.4.2 How to repair tables

The symptoms of a corrupted table are usually that queries abort unexpectedly and that you observe errors such as these:

In these cases, you must repair your tables. isamchk can usually detect and fix most things that go wrong.

The repair process involves up to four stages, described below. Before you begin, you should cd to the database directory and check the permissions of the table files. Make sure they are readable by the Unix user that mysqld runs as (and to you, since you need to access the files you are checking). If it turns out you need to modify files, they must also be writable by you.

Stage 1: Checking your tables

Run isamchk *.ISM or (isamchk -e *.ISM if you have more time). Use the -s (silent) option to suppress unnecessary information.

You have to repair only those tables for which isamchk announces an error. For such tables, proceed to Stage 2.

If you get weird errors when checking (such as out of memory errors), or if isamchk crashes, go to Stage 3.

Stage 2: Easy safe repair

First, try isamchk -r -q tbl_name (-r -q means "quick recovery mode"). This will attempt to repair the index file without touching the data file. If the data file contains everything that it should and the delete links point at the correct locations within the data file, this should work and the table is fixed. Start repairing the next table. Otherwise, use the following procedure:

  1. Make a backup of the data file before continuing.
  2. Use isamchk -r tbl_name (-r means "recovery mode"). This will remove incorrect records and deleted records from the data file and reconstruct the index file.
  3. If the preceding step fails, use isamchk --safe-recover tbl_name. Safe recovery mode uses an old recovery method that handles a few cases that regular recovery mode doesn't (but is slower).

If you get weird errors when repairing (such as out of memory errors), or if isamchk crashes, go to Stage 3.

Stage 3: Difficult repair

You should only reach this stage if the first 16K block in the index file is destroyed or contains incorrect information, or if the index file is missing. In this case, it's necessary to create a new index file. Do so as follows:

  1. Move the data file to some safe place.
  2. Use the table description file to create new (empty) data and index files:
    shell> mysql db_name
    mysql> DELETE FROM tbl_name;
    mysql> quit
    
  3. Copy the old data file back onto the newly created data file. (Don't just move the old file back onto the new file; you want to retain a copy in case something goes wrong.)

Go back to Stage 2. isamchk -r -q should work now. (This shouldn't be an endless loop).

Stage 4: Very difficult repair

You should reach this stage only if the description file has also crashed. That should never happen, because the description file isn't changed after the table is created.

  1. Restore the description file from a backup and go back to Stage 3. You can also restore the index file and go back to Stage 2. In the latter case, you should start with isamchk -r.
  2. If you don't have a backup but know exactly how the table was created, create a copy of the table in another database. Remove the new data file, then move the description and index files from the other database to your crashed database. This gives you new description and index files, but leaves the data file alone. Go back to Stage 2 and attempt to reconstruct the index file.

13.4.3 Table optimization

To coalesce fragmented records and eliminate wasted space resulting from deleting or updating records, run isamchk in recovery mode:

shell> isamchk -r tbl_name

You can optimize a table in the same way using the SQL OPTIMIZE TABLE statement. OPTIMIZE TABLE is easier, but isamchk is faster.

isamchk also has a number of other options you can use to improve the performance of a table:

-S, --sort-index
-R index_num, --sort-records=index_num
-a, --analyze

For a full description of the option see section 13.1.1 isamchk invocation syntax.

13.5 Log file maintenance

When using MySQL with log files, you will from time to time want to remove/backup old log files and tell MySQL to start logging on new files. See section 9.2 The update log.

One a Linux (Redhat) installation, you can use the mysql-log-rotate script for this. If you installed MySQL from an RPM distribution, the script should have been installed automatically.

On other systems you must install a short script yourself that you start from cron to handle log files.

You can force MySQL to start using new log files by using mysqladmin flush-logs or by using the SQL command FLUSH LOGS. If you are using MySQL 3.21 you must use mysqladmin refresh.

The above command does the following:

If you are using only an update log, you only have to flush the logs and then move away the old update log files to a backup. If you are using the normal logging, you can do something like:

shell> cd mysql-data-directory
shell> mv mysql.log mysql.old
shell> mysqladmin flush-tables

and then take a backup and remove `mysql.old'.

14 Adding new functions to MySQL

There are two ways to add new functions to MySQL:

Each method has advantages and disadvantages:

Whichever method you use to add new functions, they may be used just like native functions such as ABS() or SOUNDEX().

14.1 Adding a new user-definable function

For the UDF mechanism to work, functions must be written in C or C++ and your operating system must support dynamic loading. The MySQL source distribution includes a file `sql/udf_example.cc' that defines 5 new functions. Consult this file to see how UDF calling conventions work.

For each function that you want to use in SQL statements, you should define corresponding C (or C++) functions. In the discussion below, the name "xxx" is used for an example function name. To distinquish between SQL and C/C++ usage, XXX() (uppercase) indicates a SQL function call, and xxx() (lowercase) indicates a C/C++ function call.

The C/C++ functions that you write to implement the inferface for XXX() are:

xxx() (required)
The main function. This is where the function result is computed. The correspondence between the SQL type and return type of your C/C++ function is shown below:
SQL type C/C++ type
STRING char *
INTEGER long long
REAL double
xxx_init() (optional)
The initialization function for xxx(). It can be used to:
xxx_deinit() (optional)
The deinitialization function for xxx(). It should deallocate any memory allocated by the initialization function.

When a SQL statement invokes XXX(), MySQL calls the initialization function xxx_init() to let it perform any required setup, such as argument checking or memory allocation. If xxx_init() returns an error, the SQL statement is aborted with an error message and the main and deinitialization functions are not called. Otherwise, the main function xxx() is called once for each row. After all rows have been processed, the deinitialization function xxx_deinit() is called so it can perform any required cleanup.

All functions must be thread-safe (not just the main function, but the initialization and deinitialization functions as well). This means that you are not allowed to allocate any global or static variables that change! If you need memory, you should allocate it in xxx_init() and free it in xxx_deinit().

14.1.1 UDF calling sequences

The main function should be declared as shown below. Note that the return type and parameters differ, depending on whether you will declare the SQL function XXX() to return STRING, INTEGER or REAL in the CREATE FUNCTION statement:

For STRING functions:

char *xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *result, unsigned long *length,
              char *is_null, char *error);

For INTEGER functions:

long long xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

For REAL functions:

double xxx(UDF_INIT *initid, UDF_ARGS *args,
              char *is_null, char *error);

The initialization and deinitialization functions are declared like this:

my_bool xxx_init(UDF_INIT *initid, UDF_ARGS *args, char *message);

void xxx_deinit(UDF_INIT *initid);

The initid parameter is passed to all three functions. It points to a UDF_INIT structure that is used to communicate information between functions. The UDF_INIT structure members are listed below. The initialization function should fill in any members that it wishes to change. (To use the default for a member, leave it unchanged.)

my_bool maybe_null
xxx_init() should set maybe_null to 1 if xxx() can return NULL. The default value is 1 if any of the arguments are declared maybe_null.
unsigned int decimals
Number of decimals. The default value is the maximum number of decimals in the arguments passed to the main function. (For example, if the function is passed 1.34, 1.345 and 1.3, the default would be 3, since 1.345 has 3 decimals.
unsigned int max_length
The maximum length of the string result. The default value differs depending on the result type of the function. For string functions, the default is the length of the longest argument. For integer functions, the default is 21 digits. For real functions, the default is 13 plus the number of decimals indicated by initid->decimals. (For numeric functions, the length includes any sign or decimal point characters.)
char *ptr
A pointer that the function can use for its own purposes. For example, functions can use initid->ptr to communicate allocated memory between functions. In xxx_init(), allocate the memory and assign it to this pointer:
initid->ptr = allocated_memory;
In xxx() and xxx_deinit(), refer to initid->ptr to use or deallocate the memory.

14.1.2 Argument processing

The args parameter points to a UDF_ARGS structure which has the members listed below:

unsigned int arg_count
The number of arguments. Check this value in the initialization function if you want your function to be called with a particular number of arguments. For example:
if (args->arg_count != 2)
{
    strcpy(message,"XXX() requires two arguments");
    return 1;
}
enum Item_result *arg_type
The types for each argument. The possible type values are STRING_RESULT, INT_RESULT and REAL_RESULT. To make sure that arguments are of a given type and return an error if they are not, check the arg_type array in the initialization function. For example:
if (args->arg_type[0] != STRING_RESULT
      && args->arg_type[1] != INT_RESULT)
{
    strcpy(message,"XXX() requires a string and an integer");
    return 1;
}
As an alternative to requiring your function's arguments to be of particular types, you can use the initialization function to set the arg_type elements to the types you want. This causes MySQL to coerce arguments to those types for each call to xxx(). For example, to specify coercion of the first two arguments to string and integer, do this in xxx_init():
args->arg_type[0] = STRING_RESULT;
args->arg_type[1] = INT_RESULT;
char **args
args->args communicates information to the initialization function about the general nature of the arguments your function was called with. For a constant argument i, args->args[i] points to the argument value. (See below for instructions on how to access the value properly.) For a non-constant argument, args->args[i] is 0. A constant argument is an expression that uses only constants, such as 3 or 4*7-2 or SIN(3.14). A non-constant argument is an expression that refers to values that may change from row to row, such as column names or functions that are called with non-constant arguments. For each invocation of the main function, args->args contains the actual arguments that are passed for the row currently being processed. Functions can refer to an argument i as follows:
unsigned long *lengths
For the initialization function, the lengths array indicates the maximum string length for each argument. For each invocation of the main function, lengths contains the actual lengths of any string arguments that are passed for the row currently being processed. For arguments of types INT_RESULT or REAL_RESULT, lengths still contains the maximum length of the argument (as for the initialization function).

14.1.3 Return values and error handling

The initialization function should return 0 if no error occurred and 1 otherwise. If an error occurs, xxx_init() should store a null-terminated error message in the message parameter. The message will be returned to the client. The message buffer is MYSQL_ERRMSG_SIZE characters long, but you should try to keep the message to less than 80 characters so that it fits the width of a standard terminal screen.

The return value of the main function xxx() is the function value, for long long and double functions. For string functions, the string is returned in the result and length arguments. result is a buffer at least 255 bytes long. Set these to the contents and length of the return value. For example:

memcpy(result, "result string", 13);
*length = 13;

The string function return value normally also points to the result.

To indicate a return value of NULL in the main function, set is_null to 1:

*is_null = 1;

To indicate an error return in the main function, set the error parameter to 1:

*error = 1;

If xxx() sets *error to 1 for any row, the function value is NULL for the current row and for any subsequent rows processed by the statement in which XXX() was invoked. (xxx() will not even be called for subsequent rows.) Note: In MySQL versions prior to 3.22.10, you should set both *error and *is_null:

*error = 1;
*is_null = 1;

14.1.4 Compiling and installing user-definable functions

Files implementing UDFs must be compiled and installed on the host where the server runs. This process is described below for the example UDF file `udf_example.cc' that is included in the MySQL source distribution. This file contains the following functions:

A dynamically-loadable file should be compiled as a sharable object file, using a command something like this:

shell> gcc -shared -o udf_example.so myfunc.cc

You can easily find out the correct compiler options for your system by running this command in the `sql' directory of your MySQL source tree:

shell> make udf_example.o

You should run a compile command similar to the one that make displays, except that you should remove the -c option near the end of the line and add -o udf_example.so to the end of the line. (On some systems, you may need to leave the -c on the command.)

Once you compile a shared object containing UDFs, you must install it and tell MySQL about it. Compiling a shared object from `udf_example.cc' produces a file named something like `udf_example.so' (the exact name may vary from platform to platform). Copy this file to some directory searched by ld, such as `/usr/lib'. On many systems, you can set the LD_LIBRARY or LD_LIBRARY_PATH environment variable to point at the directory where you have your UDF function files. The dopen manual page tells you which variable you should use on your system. You should set this in mysql.server or safe_mysqld and restart mysqld.

After the library is installed, notify mysqld about the new functions with these commands:

mysql> CREATE FUNCTION metaphon RETURNS STRING SONAME "udf_example.so";
mysql> CREATE FUNCTION myfunc_double RETURNS REAL SONAME "udf_example.so";
mysql> CREATE FUNCTION myfunc_int RETURNS INTEGER SONAME "udf_example.so";
mysql> CREATE FUNCTION lookup RETURNS STRING SONAME "udf_example.so";
mysql> CREATE FUNCTION reverse_lookup RETURNS STRING SONAME "udf_example.so";

Functions can be deleted using DROP FUNCTION:

mysql> DROP FUNCTION metaphon;
mysql> DROP FUNCTION myfunc_double;
mysql> DROP FUNCTION myfunc_int;
mysql> DROP FUNCTION lookup;
mysql> DROP FUNCTION reverse_lookup;

The CREATE FUNCTION and DROP FUNCTION statements update the system table func in the mysql database. The function's name, type and shared library name are saved in the table. You must have the insert and delete privileges for the mysql database to create and drop functions.

You should not use CREATE FUNCTION to add a function that has already been created. If you need to reinstall a function, you should remove it with DROP FUNCTION and then reinstall it with CREATE FUNCTION. You would need to do this, for example, if you recompile a new version of your function, so that mysqld gets the new version. Otherwise the server will continue to use the old version.

Active functions are reloaded each time the server starts, unless you start mysqld with the --skip-grant-tables option. In this case, UDF initialization is skipped and UDFs are unavailable. (An active function is one that has been loaded with CREATE FUNCTION and not removed with DROP FUNCTION.)

14.2 Adding a new native function

The procedure for adding a new native function is described below. Note that you cannot add native functions to a binary distribution since the procedure involves modifying MySQL source code. You must compile MySQL yourself from a source distribution. Also note that if you migrate to another version of MySQL (e.g., when a new version is released), you will need to repeat the procedure with the new version.

To add a new native MySQL function, follow these steps:

  1. Add one line to `lex.h' that defines the function name in the sql_functions[] array.
  2. Add two lines to `sql_yacc.yy'. One indicates the preprocessor symbol that yacc should define (this should be added at the beginning of the file). Then define the function parameters and add an "item" with these parameters to the simple_expr parsing rule. For an example, check all occurrences of SOUNDEX in `sql_yacc.yy' to see how this is done.
  3. In `item_func.h', declare a class inheriting from Item_num_func or Item_str_func, depending on whether your function returns a number or a string.
  4. In `item_func.cc', add one of the following declarations, depending on whether you are defining a numeric or string function:
    double   Item_func_newname::val()
    longlong Item_func_newname::val_int()
    String  *Item_func_newname::Str(String *str)
    
  5. You should probably also define the following function:
    void Item_func_newname::fix_length_and_dec()
    
    This function should at least calculate max_length based on the given arguments. max_length is the maximum number of characters the function may return. This function should also set maybe_null = 0 if the main function can't return a NULL value. The function can check if any of the function arguments can return NULL by checking the arguments maybe_null variable.

All functions must be thread-safe.

For string functions, there are some additional considerations to be aware of:

15 Adding new procedures to MySQL

In MySQL, you can define a procedure in C++ that can access and modify the data in a query before sending it to a client. The modification can be done on row by row or GROUP BY level.

We have in MySQL 3.23 created an example procedure to show you what can be done.

15.1 Procedure analyse

analyse([max elements,[max memory]])

This procedure is defined in the `sql/sql_analyse.cc'. This examines the result from your query and returns an analysis of the results.

SELECT ... FROM ... WHERE ... PROCEDURE ANALYSE([max elements,[max memory]])

15.2 Writing a procedure.

For the moment, the only documentation for this is the source. :(

You can find all information about procedures by examining the following files:

16 MySQL ODBC Support

MySQL provides support for ODBC by means of the MyODBC program.

16.1 Operating systems supported by MyODBC

MyODBC is a 32-bit ODBC (2.50) level 0 driver for Windows95 and Windows NT. We hope somebody will port it to Windows 3.x.

16.2 How to report problems with MyODBC

MyODBC has been tested with Access, Admndemo.exe, C++-Builder, Centura Team Developer (formerly Gupta SQL/Windows), ColdFusion (on Solaris and NT with svc pack 5), Crystal Reports, DataJunction, Delphi, ERwin, Excel, iHTML, FileMaker Pro, FoxPro, Notes 4.5/4.6, SBSS, Perl DBD-ODBC, Paradox, Powerbuilder, Powerdesigner 32 bit, VC++ and Visual Basic.

If you know of any other application that works with MyODBC, please mail myodbc@lists.mysql.com about this!

If you encounter difficulties, we would like to have the log file from the ODBC manager (the log you get when requesting logs from ODBCADMIN) and a MyODBC log. This will help shed some light on any problems.

To get a MyODBC log, please tag the 'Trace MyODBC' option flag in the MyODBC connect/configure screen. The log will be written to file `C:\myodbc.log'. Note that you must use MYSQL.DLL and not MYSQL2.DLL for this option to work!

16.3 Programs known to work with MyODBC

Most programs should work with MyODBC, but for each of those listed below, we have tested it ourselves or gotten confirmation from some user that it works:

Program
Comment
Access
To make Access work:
DataJunction
You have to change it to output VARCHAR rather than ENUM, as it exports the latter in a manner that causes MySQL grief.
Excel
Works. Some tips:
odbcadmin
Test program for ODBC.
Delphi
You must use DBE 3.2 or newer. Set the 'Don't optimize column width' option field when connecting to MySQL. Also, here is some potentially useful delphi code that sets up both an ODBC entry and a BDE entry for MyODBC (the BDE entry requires a BDE Alias Editor which may be had for free at a Delphi Super Page near you.): (Thanks to Bryan Brunton bryan@flesherfab.com for this)
fReg:= TRegistry.Create;
  fReg.OpenKey('\Software\ODBC\ODBC.INI\DocumentsFab', True);
  fReg.WriteString('Database', 'Documents');
  fReg.WriteString('Description', ' ');
  fReg.WriteString('Driver', 'C:\WINNT\System32\myodbc.dll');
  fReg.WriteString('Flag', '1');
  fReg.WriteString('Password', ");
  fReg.WriteString('Port', ' ');
  fReg.WriteString('Server', 'xmark');
  fReg.WriteString('User', 'winuser');
  fReg.OpenKey('\Software\ODBC\ODBC.INI\ODBC Data Sources', True);
  fReg.WriteString('DocumentsFab', 'MySQL');
  fReg.CloseKey;
  fReg.Free;

  Memo1.Lines.Add('DATABASE NAME=');
  Memo1.Lines.Add('USER NAME=');
  Memo1.Lines.Add('ODBC DSN=DocumentsFab');
  Memo1.Lines.Add('OPEN MODE=READ/WRITE');
  Memo1.Lines.Add('BATCH COUNT=200');
  Memo1.Lines.Add('LANGDRIVER=');
  Memo1.Lines.Add('MAX ROWS=-1');
  Memo1.Lines.Add('SCHEMA CACHE DIR=');
  Memo1.Lines.Add('SCHEMA CACHE SIZE=8');
  Memo1.Lines.Add('SCHEMA CACHE TIME=-1');
  Memo1.Lines.Add('SQLPASSTHRU MODE=SHARED AUTOCOMMIT');
  Memo1.Lines.Add('SQLQRYMODE=');
  Memo1.Lines.Add('ENABLE SCHEMA CACHE=FALSE');
  Memo1.Lines.Add('ENABLE BCD=FALSE');
  Memo1.Lines.Add('ROWSET SIZE=20');
  Memo1.Lines.Add('BLOBS TO CACHE=64');
  Memo1.Lines.Add('BLOB SIZE=32');

  AliasEditor.Add('DocumentsFab','MySQL',Memo1.Lines);
C++Builder
Tested with BDE 3.0. The only known problem is that when the table schema changes, query fields are not updated. BDE however does not seem to recognize primary keys, only the index PRIMARY, though this has not been a problem.
Visual basic
To be able to update a table, you must define a primary key for the table.

16.4 How to fill in the various fields in the ODBC administrator program

There are three possibilities for specifying the server name on Windows95:

Example of how to fill in the "ODBC setup":

Windows DSN name:   test
Description:        This is my test database
MySql Database:     test
Server:             194.216.84.21
User:               monty
Password:           my_password
Port:

The value for the Windows DSN name field is any name that is unique in your windows ODBC setup.

You don't have to specify values for the Server, User, Password or Port fields in the ODBC setup screen. However, if you do, the values will be used as the defaults later when you attempt to make a connection. You have the option of changing the values at that time.

If the port number is not given, the default port (3306) is used.

If you specify the option Read options from C:\my.cnf, the groups client and odbc will be read from the `C:\my.cnf' file. You can use all options that are usable by mysql_options(). See section 20.4.37 mysql_options().

16.5 How to get the value of an AUTO_INCREMENT column in ODBC

A common problem is how to get the value of an automatically generated ID from an INSERT. With ODBC, you can do something like this (assuming that auto is an AUTO_INCREMENT field):

INSERT INTO foo (auto,text) VALUES(NULL,'text');
SELECT LAST_INSERT_ID();

Or, if you are just going to insert the ID into another table, you can do this:

INSERT INTO foo (auto,text) VALUES(NULL,'text');
INSERT INTO foo2 (id,text) VALUES(LAST_INSERT_ID(),'text');

For the benefit of some ODBC applications (at least Delphi and Access), the following query can be used to find a newly-inserted row:

SELECT * FROM tbl_name WHERE auto IS NULL;

17 Using MySQL with some common programs

17.1 Using MySQL with Apache

The contrib section includes programs that lets you authenticate your users from a MySQL database and also let you log your log files into a MySQL table. See section B Contributed programs.

You can change the Apache logging format to be easily readable by MySQL by putting the following into the Apache configuration file:

LogFormat \
        "\"%h\",%{%Y%m%d%H%M%S}t,%>s,\"%b\",\"%{Content-Type}o\",  \
        \"%U\",\"%{Referer}i\",\"%{User-Agent}i\""

In MySQL you can now do something like this:

LOAD DATA INFILE '/local/access_log' INTO TABLE table_name 
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"' ESCAPED BY '\\'

18 Problems and common errors

18.1 What to do if MySQL keeps crashing

All MySQL versions are tested on many platforms before they are released. This doesn't mean that there isn't any bugs in MySQL, but that if there are bugs they are very few and can be hard to find. If you have a problem, it will always help if you try to find out exactly what crashes your system as you will have a much better chance of getting this fixed quickly.

First you should try to find out whether the problem is that the mysqld daemon dies or whether your problem has to do with your client. You can check how long your mysqld server has been up by executing mysqladmin version. If mysqld has died, you may find the reason for this in the file `mysql-data-directory/'hostname'.err'.

Since it is very difficult to know why something is crashing, first try to check whether or not things that work for others crash for you. Please try the following things:

18.2 Some common errors when using MySQL

18.2.1 MySQL server has gone away error

This section also covers the related Lost connection to server during query error.

The most common reason for the MySQL server has gone away error is that the server timed out and closed the connection. By default, the server closes the connection after 8 hours if nothing has happened. You can change the time limit with by setting the wait_timeout variable when you start mysqld.

You can check that the MySQL hasn't died by executing mysqladmin version and examining the uptime.

If you have a script, you just have to issue the query again for the client to do an automatic reconnection.

You normally can get the following error codes in this case (which one you get is OS-dependent):

CR_SERVER_GONE_ERROR The client couldn't send a question to the server.
CR_SERVER_LOST The client didn't get an error when writing to the server, but it didn't get a full answer (or any answer) to the question.

You can also get these errors if you send a query to the server that is incorrect or too large. If mysqld gets a packet that is too large or out of order, it assumes that something has gone wrong with the client and closes the connection. If you need big queries (for example, if you are working with big BLOB columns), you can increase the query limit by starting mysqld with the -O max_allowed_packet=# option (default 1M). The extra memory is allocated on demand, so mysqld will use more memory only when you issue a big query or when mysqld must return a big result row!

18.2.2 Can't connect to [local] MySQL server error

A MySQL client can connect to the mysqld server in two different ways: Unix sockets, which connect through a file in the file system (default `/tmp/mysqld.sock'), or TCP/IP, which connects through a port number. Unix sockets are faster than TCP/IP but can only be used when connecting to a server on the same computer. Unix sockets are used if you don't specify a hostname or if you specify the special hostname localhost.

The error (2002) Can't connect to ... normally means that there isn't a MySQL server running on the system or that you are using a wrong socket file or TCP/IP port when trying to connect to the mysqld server.

Start by check (using ps) that there is a process running named mysqld on your server! If there isn't any mysqld process, you should start one. See section 4.15.2 Problems starting the MySQL server

If a mysqld process is running, you can check the server by trying these different connections (the port number and socket pathname might be different in your setup, of course):

shell> mysqladmin version
shell> mysqladmin variables
shell> mysqladmin -h `hostname` version variables
shell> mysqladmin -h `hostname` --port=3306 version
shell> mysqladmin -h 'ip for your host' version
shell> mysqladmin --socket=/tmp/mysql.sock version

Note the use of backquotes rather than forward quotes with the hostname command; these cause the output of hostname (i.e., the current hostname) to be substituted into the mysqladmin command.

Here are some reasons the Can't connect to local MySQL server error might occur:

If you get the error message Can't connect to MySQL server on some_hostname, you can try the following things to find out what is the problem:

18.2.3 Host '...' is blocked error

If you get a error like this:

Host 'hostname' is blocked because of many connection errors.
Unblock with 'mysqladmin flush-hosts'

This means that mysqld has gotten a lot (max_connect_errors) of connect requests from the host 'hostname' that have been interrupted in the middle. After max_connect_errors failed requests, mysqld assumes that something is wrong (like a attack from a cracker), and blocks the site from further connections until someone executes the command mysqladmin flush-hosts.

By default, mysqld blocks a host after 10 connection errors. You can easily adjust this by starting the server like this:

shell> safe_mysqld -O max_connect_errors=10000 &

Note that if you get this error message for a given host, you should first check that there isn't anything wrong with TCP/IP connections from that host. If your TCP/IP connections aren't working, it won't do you any good to increase the value of the max_connect_errors variable!

18.2.4 Out of memory error

If you issue a query and get something like the following error:

mysql: Out of memory at line 42, 'malloc.c'
mysql: needed 8136 byte (8k), memory in use: 12481367 bytes (12189k)
ERROR 2008: MySQL client ran out of memory

Note that the error refers to the MySQL client mysql. The reason for this error is simply that the client does not have enough memory to store the whole result.

To remedy the problem, first check that your query is correct. Is it reasonable that it should return so many rows? If so, you can use mysql --quick, which uses mysql_use_result() to retrieve the result set. This places less of a load on the client (but more on the server).

18.2.5 Packet too large error

When a MySQL client or the mysqld server gets a packet bigger than max_allowed_packet bytes, it issues a Packet too large error and closes the connection.

If you are using the mysql client, you may specify a bigger buffer by starting the client with mysql --set-variable=max_allowed_packet=8M.

If you are using other clients that do not allow you to specify the maximum packet size (such as DBI), you need to set the packet size when you start the server. You cau use a command-line option to mysqld to set max_allowed_packet to a larger size. For example, if you are expecting to store the full length of a BLOB into a table, you'll need to start the server with the --set-variable=max_allowed_packet=24M option.

18.2.6 The table is full error

This error occurs when an in-memory temporary table becomes larger than tmp_table_size bytes. To avoid this problem, you can use the -O tmp_table_size=# option to mysqld to increase the temporary table size, or use the SQL option SQL_BIG_TABLES before you issue the problematic query. See section 7.24 SET OPTION syntax.

You can also start mysqld with the --big-tables option. This is exactly the same as using SQL_BIG_TABLES for all queries.

18.2.7 Commands out of sync error in client

If you get Commands out of sync; You can't run this command now in your client code, you are calling client functions in the wrong order!

This can happen, for example, if you are using mysql_use_result() and try to execute a new query before you have called mysql_free_result(). It can also happen if you try to execute two queries that return data without a mysql_use_result() or mysql_store_result() in between.

18.2.8 Ignoring user error

If you get the following error:

Found wrong password for user: 'some_user@some_host'; Ignoring user

This means that when mysqld was started or when it reloaded the permissions tables, it found an entry in the user table with an invalid password. As a result, the entry is simply ignored by the permission system.

Possible causes of and fixes for this problem:

18.2.9 Table 'xxx' doesn't exist error

If you get the error Table 'xxx' doesn't exist or Can't find file: 'xxx' (errno: 2), this means that no table exists in the current database with the name xxx.

Note that as MySQL uses directories and files to store databases and tables, the database and table names are case sensitive! (On Win32 the databases and tables names are not case sensitive, but all references to a given table within a query must use the same case!)

You can check which tables you have in the current database with SHOW TABLES. See section 7.20 SHOW syntax (Get information about tables, columns,...).

18.3 How MySQL handles a full disk

When a disk full condition occurs, MySQL does the following:

To alleviate the problem, you can take the following actions:

18.4 How to run SQL commands from a text file

The mysql client typically is used interactively, like this:

shell> mysql database

However, it's also possible to put your SQL commands in a file and tell mysql to read its input from that file. To do so, create a text file `text_file' that contains the commands you wish to execute. Then invoke mysql as shown below:

shell> mysql database < text_file

You can also start your text file with a USE db_name statement. In this case, it is unnecessary to specify the database name on the command line:

shell> mysql < text_file

See section 12.1 Overview of the different MySQL programs.

18.5 Where MySQL stores temporary files

MySQL uses the value of the TMPDIR environment variable as the pathname of the directory in which to store temporary files. If you don't have TMPDIR set, MySQL uses the system default, which is normally `/tmp' or `/usr/tmp'. If the file system containing your temporary file directory is too small, you should edit safe_mysqld to set TMPDIR to point to a directory in a file system where you have enough space! You can also set the temporary directory using the --tmpdir option to mysqld.

MySQL creates all temporary files as "hidden files". This ensures that the temporary files will be removed if mysqld is terminated. The disadvantage of using hidden files is that you will not see a big temporary file that fills up the file system in which the temporary file directory is located.

When sorting (ORDER BY or GROUP BY), MySQL normally uses one or two temporary files. The maximum disk-space needed is:

(length of what is sorted + sizeof(database pointer))
* number of matched rows
* 2

sizeof(database pointer) is usually 4, but may grow in the future for really big tables.

For some SELECT queries, MySQL also creates temporary SQL tables. These are not hidden and have names of the form `SQL_*'.

ALTER TABLE and OPTIMIZE TABLE create a temporary table in the same directory as the original table.

18.6 How to protect `/tmp/mysql.sock' from being deleted

If you have problems with the fact that anyone can delete the MySQL communication socket `/tmp/mysql.sock', you can, on most versions of Unix, protect your `/tmp' file system by setting the sticky bit on it. Log in as root and do the following:

shell> chmod +s /tmp

This will protect your `/tmp' file system so that files can be deleted only by their owners or the superuser (root).

You can check if the sticky bit is set by executing ls -ld /tmp. If the last permission bit is t, the bit is set.

18.7 Access denied error

See section 6.6 How the privilege system works. And especially see section 6.13 Causes of Access denied errors.

18.8 How to run MySQL as a normal user

The MySQL server mysqld can be started and run by any user. In order to change mysqld to run as Unix user user_name, you must do the following:

  1. Stop the server if it's running (use mysqladmin shutdown).
  2. Change the database directories and files so that user_name has privileges to read and write files in them (you may need to do this as the Unix root user):
    shell> chown -R user_name /path/to/mysql/datadir
    
    If directories or files within the MySQL data directory are symlinks, you'll also need to follow those links and change the directories and files they point to. chown -R may not follow symlinks for you.
  3. Start the server as user user_name, or, if you are using MySQL 3.22 or later, start mysqld as the Unix root user and use the --user=user_name option. mysqld will switch to run as Unix user user_name before accepting any connections.
  4. If you are using the mysql.server script to start mysqld when the system is rebooted, you should edit mysql.server to use su to run mysqld as user user_name, or to invoke mysqld with the --user option. (No changes to safe_mysqld are necessary.)

At this point, your mysqld process should be running fine and dandy as the Unix user user_name. One thing hasn't changed, though: the contents of the permissions tables. By default (right after running the permissions table install script mysql_install_db), the MySQL user root is the only user with permission to access the mysql database or to create or drop databases. Unless you have changed those permissions, they still hold. This shouldn't stop you from accessing MySQL as the MySQL root user when you're logged in as a Unix user other than root; just specify the -u root option to the client program.

Note that accessing MySQL as root, by supplying -u root on the command line, has nothing to do with MySQL running as the Unix root user, or, indeed, as other Unix user. The access permissions and user names of MySQL are completely separate from Unix user names. The only connection with Unix user names is that if you don't provide a -u option when you invoke a client program, the client will try to connect using your Unix login name as your MySQL user name.

If your Unix box itself isn't secured, you should probably at least put a password on the MySQL root users in the access tables. Otherwise, any user with an account on that machine can run mysql -u root db_name and do whatever he likes.

18.9 How to reset a forgotten password.

If you have forgot the root password for MySQL, you can restore this with the following procedure.

  1. Take down the mysqld server by sending a kill (not kill -9) to the mysqld server. The pid is stored in a .pid file which is normally in the MySQL database directory:
    kill `cat /mysql-data-directory/hostname.pid`
    
  2. Restart mysqld with the --skip-grant-tables option.
  3. Connect to the mysqld server with mysql -h hostname mysql and change the password with a GRANT command. See section 7.25 GRANT and REVOKE syntax. You can also do this with mysqladmin -h hostname -u user password 'new password'
  4. Load the privilege tables with: mysqladmin -h hostname flush-tables or with the SQL command FLUSH PRIVILEGES.

18.10 Problems with file permissions

If you have problems with file permissions, for example, if mysql issues the following error message when you create a table:

ERROR: Can't find file: 'path/with/filename.frm' (Errcode: 13)

Then the environment variable UMASK might be set incorrectly when mysqld starts up. The default umask value is 0660. You can change this behavior by starting safe_mysqld as follows:

shell> UMASK=384  # = 600 in octal
shell> export UMASK
shell> /path/to/safe_mysqld &

18.11 File not found

If you get ERROR '...' not found (errno: 23), Can't open file: ... (errno: 24) or any other error with errno 23 or errno 24 from MySQL, it means that you haven't allocated enough file descriptors for MySQL. You can use the perror utility to get a description of what the error number means:

shell> perror 23
File table overflow
shell> perror 24
Too many open files

The problem here is that mysqld is trying to keep open too many files simultaneously. You can either tell mysqld not to open so many files at once, or increase the number of file descriptors available to mysqld.

To tell mysqld to keep open fewer files at a time, you can make the table cache smaller by using the -O table_cache=32 option to safe_mysqld (the default value is 64). Reducing the value of max_connections will also reduce the number of open files (the default value is 90).

To change the number of file descriptors available to mysqld, modify the safe_mysqld script. There is a commented-out line ulimit -n 256 in the script. You can remove the '#' character to uncomment this line, and change the number 256 to change the number of file descriptors available to mysqld.

ulimit can increase the number of file descriptors, but only up to the limit imposed by the operating system. If you need to increase the OS limit on the number of file descriptors available to each process, consult the documentation for your operating system.

Note that if you run the tcsh shell, ulimit will not work! tcsh will also report incorrect values when you ask for the current limits! In this case you should start safe_mysqld with sh!

18.12 Problems using DATE columns

The format of a DATE value is 'YYYY-MM-DD'. According to ANSI SQL, no other format is allowed. You should use this format in UPDATE expressions and in the WHERE clause of SELECT statements. For example:

mysql> SELECT * FROM tbl_name WHERE date >= '1997-05-05';

As a convenience, MySQL automatically converts a date to a number if the date is used in a numeric context (and vice versa). It is also smart enough to allow a "relaxed" string form when updating and in a WHERE clause that compares a date to a TIMESTAMP, DATE or a DATETIME column. (Relaxed form means that any non-numeric character may be used as the separator between parts. For example, '1998-08-15' and '1998#08#15' are equivalent.) MySQL can also convert a string containing no separators (such as '19980815'), provided it makes sense as a date.

The special date '0000-00-00' can be stored and retrieved as '0000-00-00'. When using a '0000-00-00' date through MyODBC, it will automatically be converted to NULL in MyODBC 2.50.12 and above, because ODBC can't handle this kind of date.

Since MySQL performs the conversions described above, the following statements work:

mysql> INSERT INTO tbl_name (idate) VALUES (19970505);
mysql> INSERT INTO tbl_name (idate) VALUES ('19970505');
mysql> INSERT INTO tbl_name (idate) VALUES ('97-05-05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997.05.05');
mysql> INSERT INTO tbl_name (idate) VALUES ('1997 05 05');
mysql> INSERT INTO tbl_name (idate) VALUES ('0000-00-00');

mysql> SELECT idate FROM tbl_name WHERE idate >= '1997-05-05';
mysql> SELECT idate FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT mod(idate,100) FROM tbl_name WHERE idate >= 19970505;
mysql> SELECT idate FROM tbl_name WHERE idate >= '19970505';

However, the following will not work:

mysql> SELECT idate FROM tbl_name WHERE STRCMP(idate,'19970505')=0;

STRCMP() is a string function, so it converts idate to a string and performs a string comparison. It does not convert '19970505' to a date and perform a date comparison.

Note that MySQL does no checking whether or not the date is correct. If you store an incorrect date, such as '1998-2-31', the wrong date will be stored. If the date cannot be converted to any reasonable value, a 0 is stored in the DATE field. This is mainly a speed issue and we think it is up to the application to check the dates, and not the server.

18.13 Timezone problems

If you have a problem with SELECT NOW() returning values in GMT and not your local time, you have to set the TZ environment variable to your current timezone. This should be done for the environment in which the server runs, for example in safe_mysqld or mysql.server.

18.14 Case sensitivity in searches

By default, MySQL searches are case-insensitive (although there are some character sets that are never case insensitive, such as czech). That means that if you search with col_name LIKE 'a%', you will get all column values that start with A or a. If you want to make this search case-sensitive, use something like INDEX(col_name, "A")=0 to check a prefix. Or use STRCMP(col_name, "A") = 0 if the column value must be exactly "A".

Simple comparison operations (>=, >, = , < , <=, sorting and grouping) are based on each character's "sort value". Characters with the same sort value (like E, e and 'e) are treated as the same character!

LIKE comparisons are done on the uppercase value of each character (E == e but E <> 'e)

If you want a column always to be treated in case-sensitive fashion, declare it as BINARY. See section 7.6 CREATE TABLE syntax.

If you are using Chinese data in the so-called big5 encoding, you want to make all character columns BINARY. This works because the sorting order of big5 encoding characters is based on the order of ASCII codes.

18.15 Problems with NULL values

The concept of the NULL value is a common source of confusion for newcomers to SQL, who often think that NULL is the same thing as an empty string ". This is not the case! For example, the following statements are completely different:

mysql> INSERT INTO my_table (phone) VALUES (NULL);
mysql> INSERT INTO my_table (phone) VALUES ("");

Both statements insert a value into the phone column, but the first inserts a NULL value and the second inserts an empty string. The meaning of the first can be regarded as "phone number is not known" and the meaning of the second can be regarded as "she has no phone".

In SQL, the NULL value is always false in comparison to any other value, even NULL. An expression that contains NULL always produces a NULL value unless otherwise indicated in the documentation for the operators and functions involved in the expression. All columns in the following example return NULL:

mysql> SELECT NULL,1+NULL,CONCAT('Invisible',NULL);

If you want to search for column values that are NULL, you cannot use the =NULL test. The following statement returns no rows, because expr = NULL is FALSE, for any expression:

mysql> SELECT * FROM my_table WHERE phone = NULL;

To look for NULL values, you must use the IS NULL test. The following shows how to find the NULL phone number and the empty phone number:

mysql> SELECT * FROM my_table WHERE phone IS NULL;
mysql> SELECT * FROM my_table WHERE phone = "";

In MySQL, as in many other SQL servers, you can't index columns that can have NULL values. You must declare such columns NOT NULL. Conversely, you cannot insert NULL into an indexed column.

When reading data with LOAD DATA INFILE, empty columns are updated with ". If you want a NULL value in a column, you should use \N in the text file. The literal word 'NULL' may also be used under some circumstances. See section 7.15 LOAD DATA INFILE syntax.

When using ORDER BY, NULL values are presented first. If you sort in descending order using DESC, NULL values are presented last. When using GROUP BY, all NULL values are regarded as equal.

To help with NULL handling, you can use the IS NULL and IS NOT NULL operators and the IFNULL() function.

For some column types, NULL values are handled specially. If you insert NULL into the first TIMESTAMP column of a table, the current date and time is inserted. If you insert NULL into an AUTO_INCREMENT column, the next number in the sequence is inserted.

18.16 Problems with alias

You can use alias to refer to a column in the GROUP BY, ORDER BY or in the HAVING part. Aliases can also be used to give columns more better names:

SELECT SQRT(a*b) as rt FROM table_name GROUP BY rt HAVING rt > 0;
SELECT id,COUNT(*) AS cnt FROM table_name GROUP BY id HAVING cnt > 0;
SELECT id AS "Customer identity" FROM table_name;

Note that you ANSI SQL doesn't allow you to refer to an alias in a WHERE clause. This is because that when the WHERE code is executed the column value may not yet be determinated. For example the following query is illegal:

SELECT id,COUNT(*) AS cnt FROM table_name WHERE cnt > 0 GROUP BY id;

The WHERE statement is executed to determinate which rows should be included in the GROUP BY part while HAVING is used to decide which rows from the result set should be used.

18.17 Deleting rows from related tables

As MySQL doesn't support sub-selects or use of more than one table in the DELETE statement, you should use the following approach to delete rows from 2 related tables:

  1. SELECT the rows based on some WHERE condition in the main table.
  2. DELETE the rows in the main table based on the same condition.
  3. DELETE FROM related_table WHERE related_column IN (selected_rows)

If the total number of characters in the query with related_column is more than 1,048,576 (the default value of max_allowed_packet, you should split it into smaller parts and execute multiple DELETE statements. You will probably get the fastest DELETE by only deleting 100-1000 related_column id's per time if the related_column is an index. If the related_column isn't an index, the speed is independent of the number of arguments in the IN clause.

18.18 Solving problems with no matching rows

If you have a complicated query with many tables that doesn't return any rows, you should use the following procedure to find out what is wrong with your query:

  1. Test the query with EXPLAIN and check if you can find something that is obviously wrong. See section 7.21 EXPLAIN syntax (Get information about a SELECT).
  2. Select only those fields that are used in the WHERE clause.
  3. Remove one table at a time from the query until it returns some rows. If the tables are big, it's a good idea to use LIMIT 10 with the query.
  4. Do a SELECT for the column that should have matched a row, against the table that was last removed from the query.
  5. If you are comparing FLOAT or DOUBLE columns with numbers that have decimals, you can't use =! This problem is common in most computer languages because floating point values are not exact values.
    mysql> SELECT * FROM table_name WHERE float_column=3.5;
       ->
    mysql> SELECT * FROM table_name WHERE float_column between 3.45 and 3.55;
    
    In most cases, changing the FLOAT to a DOUBLE will fix this!
  6. If you still can't find out what's wrong, create a minimal test that can be run with mysql test < query.sql that shows your problems. You can create a test file with mysqldump --quick database tables > query.sql. Take the file up in a editor, remove some insert lines (if there are too many of these) and add your select statement last in the file. Test that you still have your problem by doing:
    shell> mysqladmin create test2
    shell> mysql test2 < query.sql
    
    Post the test file using mysqlbug to mysql@lists.mysql.com.

18.19 Problems with ALTER TABLE.

If ALTER TABLE dies with an error like this:

Error on rename of './database/name.frm' to './database/B-a.frm' (Errcode: 17)

The problem may be that MySQL has crashed in a previous ALTER TABLE and there is an old table named `A-something' or `B-something' lying around. In this case, go to the MySQL data directory and delete all files that have names starting with A- or B-. (You may want to move them elsewhere instead of deleting them).

ALTER TABLE works the following way:

If something goes wrong with the renaming operation, MySQL tries to undo the changes. If something goes seriously wrong (this shouldn't happen, of course), MySQL may leave the old table as `B-xxx' but a simple rename should get your data back.

18.20 How to change the order of columns in a table

The whole point of SQL is to abstract the application from the data storage format. You should always specify the order in wish you wish to retrieve your data. For example:

SELECT col_name1, col_name2, col_name3 FROM tbl_name;

will return columns in the order col_name1, col_name2, col_name3, whereas:

SELECT col_name1, col_name3, col_name2 FROM tbl_name;

will return columns in the order col_name1, col_name3, col_name2.

You should NEVER, in an application, use SELECT * and retrieve the columns based on their position, because the order in which columns are returned CANNOT be guaranteed over time; A simple change to your database may cause your application to fail rather dramatically.

If you want to change the order of columns anyway, you can do it as follows:

  1. Create a new table with the columns in the right order.
  2. Execute INSERT INTO new_table SELECT fields-in-new_table-order FROM old_table.
  3. Drop or rename old_table
  4. ALTER TABLE new_table RENAME old_table

19 Solving some common problems with MySQL

19.1 Database replication

The most general way to replicate a database is to use the update log. See section 9.2 The update log. This requires one database that acts as a master (to which data changes are made) and one or more other databases that act as slaves. To update a slave, just run mysql < update_log. Supply host, user and password options that are appropriate for the slave database, and use the update log from the master database as input.

If you never delete anything from a table, you can use a TIMESTAMP column to find out which rows have been inserted or changed in the table since the last replication (by comparing to the time when you did the replication last time) and only copy these rows to the mirror.

It is possible to make a two-way updating system using both the update log (for deletes) and timestamps (on both sides). But in that case you must be able to handle conflicts when the same data have been changed in both ends. You probably want to keep the old version to help with deciding what has been updated.

Because replication in this case is done with SQL statements, you should not use the following functions in statements that update the database; they may not return the same value as in the original database:

All time functions are safe to use, as the timestamp is sent to the mirror if needed. LAST_INSERT_ID() is also safe to use.

19.2 Database backups

Since MySQL tables are stored as files, it is easy to do a backup. To get a consistent backup, do a LOCK TABLES on the relevant tables. See section 7.23 LOCK TABLES/UNLOCK TABLES syntax. You only need a read lock; this allows other threads to continue to query the tables while you are making a copy of the files in the database directory. If you want to make a SQL level backup, you can use SELECT INTO OUTFILE.

Another way to backup a database is to use the mysqldump program:

  1. Do a full backup of your databases:
    shell> mysqldump --tab=/path/to/some/dir --lock-tables --opt
    
    You can also simply copy all table files (`*.frm', `*.ISD' and `*.ISM' files), as long as the server isn't updating anything.
  2. Stop mysqld if it's running, then start it with the --log-update option. You will get log files with names of the form `hostname.n', where n is a number that is incremented each time you execute mysqladmin refresh or mysqladmin flush-logs, the FLUSH LOGS statement, or restart the server. These log files provide you with the information you need to replicate changes to the database that are made subsequent to the point at which you executed mysqldump.

If you have to restore something, try to recover your tables using isamchk -r first. That should work in 99.9% of all cases. If isamchk fails, try the following procedure:

  1. Restore the original mysqldump backup.
  2. Execute the following command to re-run the updates in the update logs:
    shell> ls -1 -t -r hostname.[0-9]* | xargs cat | mysql
    

ls is used to get all the log files in the right order.

You can also do selective backups with SELECT * INTO OUTFILE 'file_name' FROM tbl_name and restore with LOAD DATA INFILE 'file_name' REPLACE ... To avoid duplicate records, you need a PRIMARY KEY or a UNIQUE key in the table. The REPLACE keyword causes old records to be replaced with new ones when a new record duplicates an old record on a unique key value.

19.3 Running multiple MySQL servers on the same machine

There are circumstances when you might want to run multiple servers on the same machine. For example, you might want to test a new MySQL release while leaving your existing production setup undisturbed. Or you might be an Internet service provider that wants to provide independent MySQL installations for different customers.

If you want to run multiple servers, the easiest way is to compile the servers with different TCP/IP ports and socket files so they are not both listening to the same TCP/IP port or socket file.

Assume an existing server is configured for the default port number and socket file. Then configure the new server with a configure command something like this:

shell> ./configure  --with-tcp-port=port_number \
             --with-unix-socket=file_name \
             --prefix=/usr/local/mysql-3.22.9

Here port_number and file_name should be different than the default port number and socket file pathname, and the --prefix value should specify an installation directory different than the one under which the existing MySQL installation is located.

You can check the socket and port used by any currently-executing MySQL server with this command:

shell> mysqladmin -h hostname --port=port_number variables

If you have a MySQL server running on the port you used, you will get a list of some of the most important configurable variables in MySQL, including the socket name.

You should also edit the initialization script for your machine (probably `mysql.server') to start and kill multiple mysqld servers.

You don't have to recompile a new MySQL server just to start with a different port and socket. You can change the port and socket to be used by specifying them at runtime as options to safe_mysqld:

shell> /path/to/safe_mysqld --socket=file_name --port=port_number

If you run the new server on the same database directory as another server with logging enabled, you should also specify the name of the log files to safe_mysqld with --log and --log-update. Otherwise, both servers may be trying to write to the same log file.

Warning: Normally you should never have two servers that update data in the same database! If your OS doesn't support fault-free system locking, this may lead to unpleasant surprises!

If you want to use another database directory for the second server, you can use the --datadir=path option to safe_mysqld.

When you want to connect to a MySQL server that is running with a different port than the port that is compiled into your client, you can use one of the following methods:

20 MySQL client tools and APIs

20.1 MySQL C API

The C API code is distributed with MySQL. It is included in the mysqlclient library and allows C programs to access a database.

Many of the clients in the MySQL source distribution are written in C. If you are looking for examples that demonstrate how to use the C API, take a look at these clients.

Most of the other client APIs (all except Java) use the mysqlclient library to communicate with the MySQL server. This means that, for example, you can take advantage of many of the same environment variables that are used by other client programs, because they are referenced from the library. See section 12.1 Overview of the different MySQL programs, for a list of these variables.

The client has a maximum communication buffer size. The size of the buffer that is allocated initially (16K bytes) is automatically increased up to the maximum size (the default maximum is 24M). Since buffer sizes are increased only as demand warrants, simply increasing the default maximum limit does not in inself cause more resources to be used. This size check is mostly a check for erroneous queries and communication packets.

The communication buffer must be large enough to contain a single SQL statement (for client-to-server traffic) and one row of returned data (for server-to-client traffic). Each thread's communication buffer is dynamically enlarged to handle any query or row up to the maximum limit. For example, if you have BLOB values that contain up to 16M of data, you must have a communication buffer limit of at least 16M (in both server and client). The client's default maximum is 24M, but the default maximum in the server is 1M. You can increase this by changing the value of the max_allowed_packet parameter when the server is started. See section 10.1 Tuning server parameters.

The MySQL server shrinks each communication buffer to net_buffer_length bytes after each query. For clients, the size of the buffer associated with a connection is not decreased until the connection is closed, at which time client memory is reclaimed.

20.2 C API datatypes

MYSQL
This structure represents a handle to one database connection. It is used for almost all MySQL functions.
MYSQL_RES
This structure represents the result of a query that returns rows (SELECT, SHOW, DESCRIBE, EXPLAIN). The information returned from a query is called the result set in the remainder of this section.
MYSQL_ROW
This is a type-safe representation of one row of data. It is currently implemented as an array of counted byte strings. (You cannot treat these as null-terminated strings if field values may contain binary data, because such values may contain null bytes internally.) Rows are obtained by calling mysql_fetch_row().
MYSQL_FIELD
This structure contains information about a field, such as the field's name, type and size. Its members are described in more detail below. You may obtain the MYSQL_FIELD structures for each field by calling mysql_fetch_field() repeatedly. Field values are not part of this structure; they are contained in a MYSQL_ROW structure.
MYSQL_FIELD_OFFSET
This is a type-safe representation of an offset into a MySQL field list. (Used by mysql_field_seek().) Offsets are field numbers within a row, beginning at zero.
my_ulonglong
The type used for the number of rows and for mysql_affected_rows(), mysql_num_rows() and mysql_insert_id(). This type provides a range of 0 to 1.84e19. On some systems, attempting to print a value of type my_ulonglong will not work. To print such a value, convert it to unsigned long and use a %lu print format. Example:
printf (Number of rows: %lu\n", (unsigned long) mysql_num_rows(result));

The MYSQL_FIELD structure contains the members listed below:

char * name
The name of the field.
char * table
The name of the table containing this field, if it isn't a calculated field. For calculated fields, the table value is a NULL pointer.
char * def
The default value of this field (set only if you use mysql_list_fields()).
enum enum_field_types type
The type of the field. The type value may be one of the following:
Type value Type meaning
FIELD_TYPE_TINY TINYINT field
FIELD_TYPE_SHORT SMALLINT field
FIELD_TYPE_LONG INTEGER field
FIELD_TYPE_INT24 MEDIUMINT field
FIELD_TYPE_LONGLONG BIGINT field
FIELD_TYPE_DECIMAL DECIMAL or NUMERIC field
FIELD_TYPE_FLOAT FLOAT field
FIELD_TYPE_DOUBLE DOUBLE or REAL field
FIELD_TYPE_TIMESTAMP TIMESTAMP field
FIELD_TYPE_DATE DATE field
FIELD_TYPE_TIME TIME field
FIELD_TYPE_DATETIME DATETIME field
FIELD_TYPE_YEAR YEAR field
FIELD_TYPE_STRING String (CHAR or VARCHAR) field
FIELD_TYPE_BLOB BLOB or TEXT field (use max_length to determine the maximum length)
FIELD_TYPE_SET SET field
FIELD_TYPE_ENUM ENUM field
FIELD_TYPE_NULL NULL-type field
FIELD_TYPE_CHAR Deprecated; use FIELD_TYPE_TINY instead
You can use the IS_NUM() macro to test whether or not a field has a numeric type. Pass the type value to IS_NUM() and it will evaluate to TRUE if the field is numeric:
if (IS_NUM(field->type))
    printf("Field is numeric\n");
unsigned int length
The width of the field.
unsigned int max_length
The maximum width of the field for the result set. If you used mysql_list_fields(), this contains the maximum length for the field.
unsigned int flags
Different bit-flags for the field. The flags value may have zero or more of the following bits set:
Flag value Flag meaning
NOT_NULL_FLAG Field can't be NULL
PRI_KEY_FLAG Field is part of a primary key
UNIQUE_KEY_FLAG Field is part of a unique key
MULTIPLE_KEY_FLAG Field is part of a non-unique key.
UNSIGNED_FLAG Field has the UNSIGNED attribute
ZEROFILL_FLAG Field has the ZEROFILL attribute
BINARY_FLAG Field has the BINARY attribute
AUTO_INCREMENT_FLAG Field has the AUTO_INCREMENT attribute
ENUM_FLAG Field is an ENUM (deprecated)
BLOB_FLAG Field is a BLOB or TEXT (deprecated)
TIMESTAMP_FLAG Field is a TIMESTAMP (deprecated)
Use of the BLOB_FLAG, ENUM_FLAG and TIMESTAMP_FLAG flags is deprecated because they indicate the type of a field rather than an attribute of its type. It is preferable to test field->type against FIELD_TYPE_BLOB, FIELD_TYPE_ENUM or FIELD_TYPE_TIMESTAMP instead. The example below illustrates a typical use of the flags value:
if (field->flags & NOT_NULL_FLAG)
    printf("Field can't be null\n");
You may use the following convenience macros to determine the boolean status of the flags value:
IS_NOT_NULL(flags) True if this field is defined as NOT NULL
IS_PRI_KEY(flags) True if this field is a primary key
IS_BLOB(flags) True if this field is a BLOB or TEXT (deprecated; test field->type instead)
unsigned int decimals
The number of decimals for numeric fields.

20.3 C API function overview

The functions available in the C API are listed below and are described in greater detail in the next section. See section 20.4 C API function descriptions.

mysql_affected_rows() Returns the number of rows affected by the last UPDATE, DELETE or INSERT query.
mysql_close() Closes a server connection.
mysql_connect() Connects to a MySQL server. This function is deprecated; use mysql_real_connect() instead.
mysql_change_user() Change user and database on an open connection.
mysql_create_db() Creates a database. This function is deprecated; use the SQL command CREATE DATABASE instead.
mysql_data_seek() Seeks to an arbitrary row in a query result set.
mysql_debug() Does a DBUG_PUSH with the given string.
mysql_drop_db() Drops a database. This function is deprecated; use the SQL command DROP DATABASE instead.
mysql_dump_debug_info() Makes the server write debug information to the log.
mysql_eof() Determines whether or not the last row of a result set has been read. This function is deprecated; mysql_errno() or mysql_error() may be used instead.
mysql_errno() Returns the error number for the most recently invoked MySQL function.
mysql_error() Returns the error message for the most recently invoked MySQL function.
mysql_escape_string() Escapes special characters in a string for use in a SQL statement.
mysql_fetch_field() Returns the type of the next table field.
mysql_fetch_field_direct() Returns the type of a table field, given a field number.
mysql_fetch_fields() Returns an array of all field structures.
mysql_fetch_lengths() Returns the lengths of all columns in the current row.
mysql_fetch_row() Fetches the next row from the result set.
mysql_field_seek() Puts the column cursor on a specified column.
mysql_field_count() Returns the number of result columns for the most recent query.
mysql_field_tell() Returns the position of the field cursor used for the last mysql_fetch_field().
mysql_free_result() Frees memory used by a result set.
mysql_get_client_info() Returns client version information.
mysql_get_host_info() Returns a string describing the connection.
mysql_get_proto_info() Returns the protocol version used by the connection.
mysql_get_server_info() Returns the server version number.
mysql_info() Returns information about the most recently executed query.
mysql_init() Gets or initializes a MYSQL structure.
mysql_insert_id() Returns the ID generated for an AUTO_INCREMENT column by the previous query.
mysql_kill() Kill a given thread.
mysql_list_dbs() Returns database names matching a simple regular expression.
mysql_list_fields() Returns field names matching a simple regular expression.
mysql_list_processes() Returns a list of the current server threads.
mysql_list_tables() Returns table names matching a simple regular expression.
mysql_num_fields() Returns the number of columns in a result set.
mysql_num_rows() Returns the number of rows in a result set.
mysql_options() Set connect options for mysql_connect().
mysql_ping() Checks whether or not the connection to the server is working, reconnecting as necessary.
mysql_query() Executes a SQL query specified as a null-terminated string.
mysql_real_connect() Connects to a MySQL server.
mysql_real_query() Executes a SQL query specified as a counted string.
mysql_reload() Tells the server to reload the grant tables.
mysql_row_seek() Seeks to a row in a result set, using value returned from mysql_row_tell().
mysql_row_tell() Returns the row cursor position.
mysql_select_db() Connects to a database.
mysql_shutdown() Shuts down the database server.
mysql_stat() Returns the server status as a string.
mysql_store_result() Retrieves a complete result set to the client.
mysql_thread_id() Returns the current thread ID.
mysql_use_result() Initiates a row-by-row result set retrieval.

To connect to the server, call mysql_init() to initialize a connection handler, then call mysql_real_connect() with that handler (along with other information such as the hostname, user name and password). When you are done with the connection, call mysql_close() to terminate it.

While a connection is active, the client may send SQL queries to the server using mysql_query() or mysql_real_query(). The difference between the two is that mysql_query() expects the query to be specified as a null-terminated string whereas mysql_real_query() expects a counted string. If the string contains binary data (which may include null bytes), you must use mysql_real_query().

For each non-SELECT query (e.g., INSERT, UPDATE, DELETE), you can found out how many rows were affected (changed) by calling mysql_affected_rows().

For SELECT queries, you retrieve the selected rows as a result set. (Note that some statements are SELECT-like in that they return rows. These include SHOW, DESCRIBE and EXPLAIN. They should be treated the same way as SELECT statements.)

There are two ways for a client to process result sets. One way is to retrieve the entire result set all at once by calling mysql_store_result(). This function acquires from the server all the rows returned by the query and stores them in the client. The second way is for the client to initiate a row-by-row result set retrieval by calling mysql_use_result(). This function initializes the retrieval, but does not actually get any rows from the server.

In both cases, you access rows by calling mysql_fetch_row(). With mysql_store_result(), mysql_fetch_row() accesses rows that have already been fetched from the server. With mysql_use_result(), mysql_fetch_row() actually retrieves the row from the server. Information about as the size of the data values in each row is available by calling mysql_fetch_lengths().

After you are done with a result set, call mysql_free_result() to free the memory used for it.

The two retrieval mechanisms are complementary. Client programs should choose the approach that is most appropriate for their requirements. In practice, clients tend to use mysql_store_result() more commonly.

An advantage of mysql_store_result() is that since the rows have all been fetched to the client, you not only can access rows sequentially, you can move back and forth in the result set using mysql_data_seek() or mysql_row_seek() to change the current row position within the result set. You can also find out how many rows there are by calling mysql_num_rows(). On the other hand, the memory requirements for mysql_store_result() may be very high for large result sets and you are more likely to encounter out-of-memory conditions.

An advantage of mysql_use_result() is that the client requires less memory for the result set since it maintains only one row at a time (and since there is less allocation overhead, mysql_use_result() can be faster). Disadvantages are that you must process each row quickly to avoid tying up the server, you don't have random access to rows within the result set (you can only access rows sequentially), and you don't know how many rows are in the result set until you have retrieved them all. Furthermore, you must retrieve all the rows even if you determine in mid-retrieval that you've found the information you were looking for.

The API makes it possible for clients to respond appropriately to queries (retrieving rows only as necessary) without knowing whether or not the query is a SELECT. You can do this by calling mysql_store_result() after each mysql_query() (or mysql_real_query()). If the result set call succeeds, the query was a SELECT and you can read the rows. If the result set call fails, call mysql_field_count() to determine whether or not a result was actually to be expected. If mysql_field_count() returns zero, the query returned no data (indicating that it was an INSERT, UPDATE, DELETE, etc.), and thus not expected to return rows. If mysql_field_count() is non-zero, the query should have returned rows, but didn't. This indicates that the query was a SELECT that failed. See the description for mysql_field_count() for an example of how this can be done.

Both mysql_store_result() and mysql_use_result() allow you to obtain information about the fields that make up the result set (the number of fields, their names and types, etc.). You can access field information sequentially within the row by calling mysql_fetch_field() repeatedly, or by field number within the row by calling mysql_fetch_field_direct(). The current field cursor position may be changed by calling mysql_field_seek(). Setting the field cursor affects subsequent calls to mysql_fetch_field(). You can also get information for fields all at once by calling mysql_fetch_fields().

For detecting and reporting errors, MySQL provides access to error information by means of the mysql_errno() and mysql_error() functions. These return the error code or error message for the most recently invoked function that can succeed or fail, allowing you to determine when an error occurred and what it was.

20.4 C API function descriptions

In the descriptions below, a parameter or return value of NULL means NULL in the sense of the C programming language, not a MySQL NULL value.

Functions that return a value generally return a pointer or an integer. Unless specified otherwise, functions returning a pointer return a non-NULL value to indicate success or a NULL value to indicate an error, and functions returning an integer return zero to indicate success or non-zero to indicate an error. Note that "non-zero" means just that. Unless the function description says otherwise, do not test against a value other than zero:

if (result)                   /* correct */
    ... error ...

if (result < 0)               /* incorrect */
    ... error ...

if (result == -1)             /* incorrect */
    ... error ...

When a function returns an error, the Errors subsection of the function description lists the possible types of errors. You can find out which of these occurred by calling mysql_errno(). A string representation of the error may be obtained by calling mysql_error().

20.4.1 mysql_affected_rows()

my_ulonglong mysql_affected_rows(MYSQL *mysql)

20.4.1.1 Description

Returns the number of rows affected (changed) by the last UPDATE, DELETE or INSERT query. May be called immediately after mysql_query() for UPDATE, DELETE or INSERT statements. For SELECT statements, mysql_affected_rows() works like mysql_num_rows().

mysql_affected_rows() is currently implemented as a macro.

20.4.1.2 Return values

An integer greater than zero indicates the number of rows affected or retrieved. Zero indicates that no records matched the WHERE clause in the query or that no query has yet been executed. -1 indicates that the query returned an error or that, for a SELECT query, mysql_affected_rows() was called prior to calling mysql_store_result().

20.4.1.3 Errors

None.

20.4.1.4 Example

mysql_query(&mysql,"UPDATE products SET cost=cost*1.25 WHERE group=10");
printf("%d products updated",mysql_affected_rows(&mysql));

20.4.2 mysql_close()

void mysql_close(MYSQL *mysql)

20.4.2.1 Description

Closes a previously opened connection. mysql_close() also deallocates the connection handle pointed to by mysql if the handle was allocated automatically by mysql_init() or mysql_real_connect().

20.4.2.2 Return values

None.

20.4.2.3 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

20.4.3 mysql_connect()

MYSQL *mysql_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd)

20.4.3.1 Description

This function is deprecated. It is preferable to use mysql_real_connect() instead.

mysql_connect() attempts to establish a connection to a MySQL database engine running on host. mysql_connect() must complete successfully before you can execute any of the other API functions, with the exception of mysql_get_client_info().

The meanings of the parameters are the same as for the corresponding parameters for mysql_real_connect(). See the description of that function for more information.

20.4.3.2 Return values

Same as for mysql_real_connect().

20.4.3.3 Errors

Same as for mysql_real_connect().

20.4.4 mysql_change_user()

my_bool mysql_change_user(MYSQL *mysql, const char *user, const char *password, const char *db)

20.4.4.1 Description

Changes the user and causes the database specified by db to become the default (current) database on the connection specified by mysql. In subsequent queries, this database is the default for table references that do not include an explicit database specifier.

This function was introduced in MySQL 3.23.3

mysql_cohange_user() fails unless the connected user can be authenticated or if he doesn't have permission to use the database. In this case the user and database is not changed

20.4.4.2 Return values

Zero for success. Non-zero if an error occurred.

20.4.4.3 Errors

The same that you can get from mysql_real_connect()

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.
ER_UNKNOWN_COM_ERROR
The MySQL server doesn't implement this command (probably an old server)
ER_ACCESS_DENIED_ERROR
The user or password was wrong.
ER_BAD_DB_ERROR
The database didn't exists.
ER_DBACCESS_DENIED_ERROR
The user did not have access rights to the database.
ER_WRONG_DB_NAME
The database name was too long.

20.4.4.4 Example

if (mysql_change_user(&mysql, "user", "password", "new_database"))
{
   fprintf(stderr, "Failed to change user.  Error: %s\n",
           mysql_error(&mysql));
}

20.4.5 mysql_create_db()

int mysql_create_db(MYSQL *mysql, const char *db)

20.4.5.1 Description

Creates the database named by the db parameter.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL CREATE DATABASE statement instead.

20.4.5.2 Return values

Zero if the database was created successfully. Non-zero if an error occurred.

20.4.5.3 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

20.4.5.4 Example

if(mysql_create_db(&mysql, "my_database"))
{
   fprintf(stderr, "Failed to create new database.  Error: %s\n",
           mysql_error(&mysql));
}

20.4.6 mysql_data_seek()

void mysql_data_seek(MYSQL_RES *result, unsigned int offset)

20.4.6.1 Description

Seeks to an arbitrary row in a query result set. This requires that the result set structure contains the entire result of the query, so mysql_data_seek() may be used in conjunction only with mysql_store_result(), not with mysql_use_result().

The offset should be a value in the range from 0 to mysql_num_rows(result)-1.

20.4.6.2 Return values

None.

20.4.6.3 Errors

None.

20.4.7 mysql_debug()

void mysql_debug(char *debug)

20.4.7.1 Description

Does a DBUG_PUSH with the given string. mysql_debug() uses the Fred Fish debug library. To use this function, you must compile the client library to support debugging. See section G.1 Debugging a MySQL server. See section G.2 Debugging a MySQL client.

20.4.7.2 Return values

None.

20.4.7.3 Errors

None.

20.4.7.4 Example

The call shown below causes the client library to generate a trace file in `/tmp/client.trace' on the client machine:

mysql_debug("d:t:O,/tmp/client.trace");

20.4.8 mysql_drop_db()

int mysql_drop_db(MYSQL *mysql, const char *db)

20.4.8.1 Description

Drops the database named by the db parameter.

This function is deprecated. It is preferable to use mysql_query() to issue a SQL DROP DATABASE statement instead.

20.4.8.2 Return values

Zero if the database was dropped successfully. Non-zero if an error occurred.

20.4.8.3 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

20.4.8.4 Example

if(mysql_drop_db(&mysql, "my_database"))
  fprintf(stderr, "Failed to drop the database: Error: %s\n",
          mysql_error(&mysql));

20.4.9 mysql_dump_debug_info()

int mysql_dump_debug_info(MYSQL *mysql)

20.4.9.1 Description

Instructs the server to write some debug information to the log. The connected user must have the process privilege for this to work.

20.4.9.2 Return values

Zero if the command was successful. Non-zero if an error occurred.

20.4.9.3 Errors

CR_COMMANDS_OUT_OF_SYNC
Commands were executed in an improper order.
CR_SERVER_GONE_ERROR
The MySQL server has gone away.
CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

20.4.10 mysql_eof()

my_bool mysql_eof(MYSQL_RES *result)

20.4.10.1 Description

This function is deprecated. mysql_errno() or mysql_error() may be used instead.

mysql_eof() determines whether or not the last row of a result set has been read.

If you acquire a result set from a successful call to mysql_store_result(), the client receives the entire set in one operation. In this case, a NULL return from mysql_fetch_row() always means the end of the result set has been reached and it is unnecessary to call mysql_eof().

On the other hand, if you use mysql_use_result() to initiate a result set retrieval, the rows of the set are obtained from the server one by one as you call mysql_fetch_row() repeatedly. Because an error may occur on the connection during this process, a NULL return value from mysql_fetch_row() does not necessarily mean the end of the result set was reached normally. In this case, you can use mysql_eof() to determine what happened. mysql_eof() returns a non-zero value if the end of the result set was reached and zero if an error occurred.

Historically, mysql_eof() predates the standard MySQL error functions mysql_errno() and mysql_error(). Since those error functions provide the same information, their use is preferred over mysql_eof(), which is now deprecated. (In fact, they provide more information, since mysql_eof() returns only a boolean value whereas the error functions indicate a reason for the error when one occurs.)

20.4.10.2 Return values

Zero if an error occurred. Non-zero if the end of the result set has been reached.

20.4.10.3 Errors

None.

20.4.10.4 Example

The following example shows how you might use mysql_eof():

mysql_query(&mysql,"SELECT * FROM some_table");
result = mysql_use_result(&mysql);
while((row = mysql_fetch_row(result)))
{
    // do something with data
}
if(!mysql_eof(result))  // mysql_fetch_row() failed due to an error
{
    fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
}

However, you can achieve the same effect with the standard MySQL error functions:

mysql_query(&mysql,"SELECT * FROM some_table");
result = mysql_use_result(&mysql);
while((row = mysql_fetch_row(result)))
{
    // do something with data
}
if(mysql_errno(&mysql))  // mysql_fetch_row() failed due to an error
{
    fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
}

20.4.11 mysql_errno()

unsigned int mysql_errno(MYSQL *mysql)

20.4.11.1 Description

For the connection specified by mysql, mysql_errno() returns the error code for the most recently invoked API function that can succeed or fail. A return value of zero means that no error occurred. Client error message numbers are listed in the MySQL `errmsg.h' header file. Server error message numbers are listed in `mysqld_error.h'

20.4.11.2 Return values:

An error code value. Zero if no error occurred.

20.4.11.3 Errors

None.

20.4.12 mysql_error()

char *mysql_error(MYSQL *mysql)

20.4.12.1 Description

For the connection specified by mysql, mysql_error() returns the error message for the most recently invoked API function that can succeed or fail. An empty string ("") is returned if no error occurred. This means the following two tests are equivalent:

if(mysql_errno(&mysql))
{
    // an error occurred
}

if(mysql_error(&mysql)[0] != '\0')
{
    // an error occurred
}

The language of the client error messages may be changed by recompiling the MySQL client library. Currently you can choose error messages in several different languages. See section 9.1 What languages are supported by MySQL?.

20.4.12.2 Return values

A character string that describes the error. An empty string if no error occurred.

20.4.12.3 Errors

None.

20.4.13 mysql_escape_string()

unsigned int mysql_escape_string(char *to, const char *from, unsigned int length)

20.4.13.1 Description

Encodes the string in from to an escaped SQL string that can be sent to the server in a SQL statement, and places the result in to. Characters encoded are NUL (ASCII 0), `\n', `\r', `\' and `'' (see section 7.1 Literals: how to write strings and numbers).

The string pointed to by from must be length bytes long (not including the terminating null byte). You must allocate the to buffer to be at least length*2+1 bytes long. When mysql_escape_string() returns, the contents of to will be a null-terminated string. The return value is the length of the encoded string, not including the terminating null character.

20.4.13.2 Example

char query[1000],*end;

end = strmov(query,"INSERT INTO test_table values(");
*end++ = '\";
end += mysql_escape_string(end,"What's this",11);
*end++ = '\";
*end++ = ',';
*end++ = '\";
end += mysql_escape_string(end,"binary data: \0\r\n",16);
*end++ = '\";
*end++ = ')';

if (mysql_real_query(&mysql,query,(unsigned int) (end - query)))
{
   fprintf(stderr, "Failed to insert row, Error: %s\n",
           mysql_error(&mysql));
}

The strmov() function used in the example is included in the mysqlclient library and works like strcpy() but returns a pointer to the terminating null of the first parameter.

20.4.13.3 Return values

The length of the value placed into to, not including the terminating null character.

20.4.13.4 Errors

None.

20.4.14 mysql_fetch_field()

MYSQL_FIELD *mysql_fetch_field(MYSQL_RES *result)

20.4.14.1 Description

Returns the definition of one column of a result set as a MYSQL_FIELD structure. Call this function repeatedly to retrieve information about all columns in the result set. mysql_fetch_field() returns NULL when no more fields are left.

mysql_fetch_field() is reset to return information about the first field each time you execute a new SELECT query. The field returned by mysql_fetch_field() is also affected by calls to mysql_field_seek().

If you've called mysql_query() to perform a SELECT on a table but have not called mysql_store_result(), MySQL returns the default blob length (8K bytes) if you call mysql_fetch_field() to ask for the length of a BLOB field. (The 8K size is chosen because MySQL doesn't know the maximum length for the BLOB. This should be made configurable sometime.) Once you've retrieved the result set, field->max_length contains the length of the largest value for this column in the specific query.

20.4.14.2 Return values

The MYSQL_FIELD structure for the current column. NULL if no columns are left.

20.4.14.3 Errors

None.

20.4.14.4 Example

MYSQL_FIELD *field;

while((field = mysql_fetch_field(result)))
{
    printf("field name %s\n", field->name);
}

20.4.15 mysql_fetch_fields()

MYSQL_FIELD *mysql_fetch_fields(MYSQL_RES *result)

20.4.15.1 Description

Returns an array of all MYSQL_FIELD structures for a result set. Each structure provides the field definition for one column of the result set.

20.4.15.2 Return values

An array of MYSQL_FIELD structures for all columns of a result set.

20.4.15.3 Errors

None.

20.4.15.4 Example

unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *fields;

num_fields = mysql_num_fields(result);
fields = mysql_fetch_fields(result);
for(i = 0; i < num_fields; i++)
{
   printf("Field %u is %s\n", i, fields[i].name);
}

20.4.16 mysql_fetch_field_direct()

MYSQL_FIELD *mysql_fetch_field_direct(MYSQL_RES *result, unsigned int fieldnr)

20.4.16.1 Description

Given a field number fieldnr for a column within a result set, returns that column's field definition as a MYSQL_FIELD structure. You may use this function to retrieve the definition for an arbitrary column. The value of fieldnr should be in the range from 0 to mysql_num_fields(result)-1.

20.4.16.2 Return values

The MYSQL_FIELD structure for the specified column.

20.4.16.3 Errors

None.

20.4.16.4 Example

unsigned int num_fields;
unsigned int i;
MYSQL_FIELD *field;

num_fields = mysql_num_fields(result);
for(i = 0; i < num_fields; i++)
{
    field = mysql_fetch_field_direct(result, i);
    printf("Field %u is %s\n", i, field->name);
}

20.4.17 mysql_fetch_lengths()

unsigned long *mysql_fetch_lengths(MYSQL_RES *result)

20.4.17.1 Description

Returns the lengths of the columns of the current row within a result set. If you plan to copy field values, this length information is also useful for optimization, because you can avoid calling strlen(). In addition, if the result set contains binary data, you must use this function to determine the size of the data, because strlen() returns incorrect results for any field containing null characters.

The length for empty columns and for columns containing NULL values is zero. To see how to distinguish these two cases, see the description for mysql_fetch_row().

20.4.17.2 Return values

An array of unsigned long integers representing the size of each column (not including any terminating null characters). NULL if an error occurred.

20.4.17.3 Errors

mysql_fetch_lengths() is valid only for the current row of the result set. It returns NULL if you call it before calling mysql_fetch_row() or after retrieving all rows in the result.

20.4.17.4 Example

MYSQL_ROW row;
unsigned long *lengths;
unsigned int num_fields;
unsigned int i;

row = mysql_fetch_row(result);
if (row)
{
    num_fields = mysql_num_fields(result);
    lengths = mysql_fetch_lengths(result);
    for(i = 0; i < num_fields; i++)
    {
         printf("Column %u is %lu bytes in length.\n", i, lengths[i]);
    }
}

20.4.18 mysql_fetch_row()

MYSQL_ROW mysql_fetch_row(MYSQL_RES *result)

20.4.18.1 Description

Retrieves the next row of a result set. When used after mysql_store_result(), mysql_fetch_row() returns NULL when there are no more rows to retrieve. When used after mysql_use_result(), mysql_fetch_row() returns NULL when there are no more rows to retrieve or if an error occurred.

The number of values in the row is given by mysql_num_fields(result). If row holds the return value from a call to mysql_fetch_row(), pointers to the values are accessed as row[0] to row[mysql_num_fields(result)-1]. NULL values in the row are indicated by NULL pointers.

The lengths of the field values in the row may be obtained by calling mysql_fetch_lengths(). Empty fields and fields containing NULL both have length 0; you can distinguish these by checking the pointer for the field value. If the pointer is NULL, the field is NULL; otherwise the field is empty.

20.4.18.2 Return values

A MYSQL_ROW structure for the next row. NULL if there are no more rows to retrieve or if an error occurred.

20.4.18.3 Errors

CR_SERVER_LOST
The connection to the server was lost during the query.
CR_UNKNOWN_ERROR
An unknown error occurred.

20.4.18.4 Example

MYSQL_ROW row;
unsigned int num_fields;
unsigned int i;

num_fields = mysql_num_fields(result);
while ((row = mysql_fetch_row(result)))
{
   unsigned long *lengths;
   lengths = mysql_fetch_lengths(result);
   for(i = 0; i < num_fields; i++)
   {
       printf("[%.*s] ", (int) lengths[i], row[i] ? row[i] : "NULL");
   }
   printf("\n");
}

20.4.19 mysql_field_count()

unsigned int mysql_field_count(MYSQL *mysql)

If you are using a version of MySQL earlier than 3.22.24, you should use unsigned int mysql_num_fields(MYSQL *mysql) instead.

20.4.19.1 Description

Returns the number of columns for the most recent query on the connection.

The normal use of this function is when mysql_store_result() returned NULL (and thus you have no result set pointer). In this case, you can call mysql_field_count() to determine whether or not mysql_store_result() should have produced a non-empty result. This allows the client program to take proper action without knowing whether or not the query was a SELECT (or SELECT-like) statement. The example shown below illustrates how this may be done.

See section 20.4.51 Why is it that after mysql_query() returns success, mysql_store_result() sometimes returns NULL?.

20.4.19.2 Return values

An unsigned integer representing the number of fields in a result set.

20.4.19.3 Errors

None.

20.4.19.4 Example

MYSQL_RES *result;
unsigned int num_fields;
unsigned int num_rows;

if (mysql_query(&mysql,query_string))
{
    // error
}
else // query succeeded, process any data returned by it
{
    result = mysql_store_result(&mysql);
    if (result)  // there are rows
    {
        num_fields = mysql_num_fields(result);
        // retrieve rows, then call mysql_free_result(result)
    }
    else  // mysql_store_result() returned nothing; should it have?
    {
        if(mysql_field_count(&mysql) == 0)
        {
            // query does not return data
            // (it was not a SELECT)
            num_rows = mysql_affected_rows(&mysql);
        }
        else // mysql_store_result() should have returned data
        {
            fprintf(stderr, "Error: %s\n", mysql_error(&mysql));
        }
    }
}

An alternative is to replace the mysql_field_count(&mysql) call with mysql_errno(&mysql). In this case, you are checking directly for an error from mysql_store_result() rather than inferring from the value of mysql_field_count() whether or not the statement was a SELECT.

20.4.20 mysql_field_seek()

MYSQL_FIELD_OFFSET mysql_field_seek(MYSQL_RES *result, MYSQL_FIELD_OFFSET offset)

20.4.20.1 Description

Sets the field cursor to the given offset. The next call to mysql_fetch_field() will retrieve the field definition of the column associated with that offset.

To seek to the beginning of a row, pass an offset value of zero.

20.4.20.2 Return values

The previous value of the field cursor.

20.4.20.3 Errors

None.

20.4.21 mysql_field_tell()

MYSQL_FIELD_OFFSET mysql_field_tell(MYSQL_RES *result)

20.4.21.1 Description

Returns the position of the field cursor used for the last mysql_fetch_field(). This value can be used as an argument to mysql_field_seek().

20.4.21.2 Return values

The current offset of the field cursor.

20.4.21.3 Errors

None.

20.4.22 mysql_free_result()

void mysql_free_result(MYSQL_RES *result)

20.4.22.1 Description

Frees the memory allocated for a result set by mysql_store_result(), mysql_use_result(), mysql_list_dbs(), etc. When you are done with a result set, you must free the memory it uses by calling mysql_free_result().

20.4.22.2 Return values

None.

20.4.22.3 Errors

None.

20.4.23 mysql_get_client_info()

char *mysql_get_client_info(void)

20.4.23.1 Description

Returns a string that represents the client library version.

20.4.23.2 Return values

A character string that represents the MySQL client library version.

20.4.23.3 Errors

None.

20.4.24 mysql_get_host_info()

char *mysql_get_host_info(MYSQL *mysql)

20.4.24.1 Description

Returns a string describing the type of connection in use, including the server host name.

20.4.24.2 Return values

A character string representing the server host name and the connection type.

20.4.24.3 Errors

None.

20.4.25 mysql_get_proto_info()

unsigned int mysql_get_proto_info(MYSQL *mysql)

20.4.25.1 Description

Returns the protocol version used by current connection.

20.4.25.2 Return values

An unsigned integer representing the protocol version used by the current connection.

20.4.25.3 Errors

None.

20.4.26 mysql_get_server_info()

char *mysql_get_server_info(MYSQL *mysql)

20.4.26.1 Description

Returns a string that represents the server version number.

20.4.26.2 Return values

A character string that represents the server version number.

20.4.26.3 Errors

None.

20.4.27 mysql_info()

char *mysql_info(MYSQL *mysql)

20.4.27.1 Description

Retrieves a string providing information about the most recently executed query, but only for the statements listed below. For other statements, mysql_info() returns NULL. The format of the string varies depending on the type of query, as described below. The numbers are illustrative only; the string will contain values appropriate for the query.

INSERT INTO ... SELECT ...
String format: Records: 100 Duplicates: 0 Warnings: 0
INSERT INTO ... VALUES (...),(...),(...)...
String format: Records: 3 Duplicates: 0 Warnings: 0
LOAD DATA INFILE ...
String format: Records: 1 Deleted: 0 Skipped: 0 Warnings: 0
ALTER TABLE
String format: Records: 3 Duplicates: 0 Warnings: 0

Note that mysql_info() returns a non-NULL value for the INSERT ... VALUES statement only if multiple value lists are specified in the statement.

20.4.27.2 Return values

A character string representing additional information about the most recently executed query. NULL if no information is available for the query.

20.4.27.3 Errors

None.

20.4.28 mysql_init()

MYSQL *mysql_init(MYSQL *mysql)

20.4.28.1 Description

Allocates or initializes a MYSQL object suitable for mysql_real_connect(). If mysql is a NULL pointer, the function allocates, initializes and returns a new object. Otherwise the object is initialized and the address of the object is returned. If mysql_init() allocates a new object, it will be freed when mysql_close() is called to close the connection.

20.4.28.2 Return values

An initialized MYSQL* handle. NULL if there was insufficient memory to allocate a new object.

20.4.28.3 Errors

In case of insufficient memory, NULL is returned.

20.4.29 mysql_insert_id()

my_ulonglong mysql_insert_id(MYSQL *mysql)

20.4.29.