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1. General Information

The MySQL (R) software delivers a very fast, multi-threaded, multi-user, and robust SQL (Structured Query Language) database server. MySQL Server is intended for mission-critical, heavy-load production systems as well as for embedding into mass-deployed software. MySQL is a trademark of MySQL AB.

The MySQL software is Dual Licensed. Users can choose to use the MySQL software as an Open Source/Free Software product under the terms of the GNU General Public License (http://www.gnu.org/licenses/) or can purchase a standard commercial license from MySQL AB. See section MySQL Support and Licensing.

The MySQL web site (http://www.mysql.com/) provides the latest information about the MySQL software.

The following list describes some sections of particular interest in this manual:


Reports of errors (often called bugs), as well as questions and comments, should be sent to the general MySQL mailing list. See section The MySQL Mailing Lists. See section How to Report Bugs or Problems.

The mysqlbug script should be used to generate bug reports on Unix. (Windows distributions contain a file `mysqlbug.txt' in the base directory that can be used as a template for a bug report.)

For source distributions, the mysqlbug script can be found in the `scripts' directory. For binary distributions, mysqlbug can be found in the `bin' directory (`/usr/bin' for the MySQL-server RPM package).

If you have found a sensitive security bug in MySQL Server, you should send an e-mail to security@mysql.com.

1.1 About This Manual

This is the MySQL reference manual; it documents MySQL up to Version 3.23.58. Functional changes are always indicated with reference to the version, so this manual is also suitable if you are using an older version of the MySQL software (such as 3.23 or 4.0-production). There are also references for version 5.0 (development).

Being a reference manual, it does not provide general instruction on SQL or relational database concepts.

As the MySQL Database Software is under constant development, the manual is also updated frequently. The most recent version of this manual is available at http://www.mysql.com/documentation/ in many different formats, including HTML, PDF, and Windows HLP versions.

The primary document is the Texinfo file. The HTML version is produced automatically using 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 pdftex.

If you have a hard time finding information in the manual, you can try our searchable version at http://www.mysql.com/doc/.

If you have any suggestions concerning additions or corrections to this manual, please send them to the documentation team at docs@mysql.com.

This manual was initially written by David Axmark and Michael (Monty) Widenius. It is now maintained by the MySQL Documentation Team, consisting of Arjen Lentz, Paul DuBois and Stefan Hinz. For the many other contributors, see Credits.

The copyright (2003) to this manual is owned by the Swedish company MySQL AB. See section Copyrights and Licenses Used by MySQL.

1.1.1 Conventions Used in This Manual

This manual uses certain typographical conventions:


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."


Constant-width font with surrounding quotes is used for filenames and pathnames. Example: "The distribution is installed under the `/usr/local/' directory."


Constant-width font with surrounding quotes is also used to indicate character sequences. Example: "To specify a wildcard, use the `%' character."


Italic font is used for emphasis, like this.


Boldface font is used in table headings 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 a prompt shown before 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 program:

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 must often 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 keywords are not case-sensitive and may be written in uppercase or lowercase. This manual uses uppercase.

In syntax descriptions, square brackets (`[' and `]') are used to indicate optional words or clauses. For example, in the following statement, IF EXISTS is optional:


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 (`[' and `]'):


When one member from a set of choices must be chosen, the alternatives are listed within braces (`{' and `}'):

{DESCRIBE | DESC} tbl_name {col_name | wild}

1.2 What Is MySQL?

MySQL, the most popular Open Source SQL database, is developed, distributed, and supported by MySQL AB. MySQL AB is a commercial company, founded by the MySQL developers, that builds its business providing services around the MySQL database. See section What Is MySQL AB?.

The MySQL web site (http://www.mysql.com/) provides the latest information about MySQL software and MySQL AB.

MySQL is a database management system.

A database is a structured collection of data. It may be anything from a simple shopping list to a picture gallery or the vast amounts of information in a corporate network. To add, access, and process data stored in a computer database, you need a database management system such as MySQL Server. Since computers are very good at handling large amounts of data, database management systems play a central role in computing, as stand-alone utilities or as parts of other applications.

MySQL is a relational database management system.

A relational database stores data in separate tables rather than putting all the data in one big storeroom. This adds speed and flexibility. The SQL part of "MySQL" stands for "Structured Query Language". SQL is the most common standardised language used to access databases and is defined by the ANSI/ISO SQL Standard.(The SQL standard has been evolving since 1986 and several versions exist. In this manual, "SQL-92" refers to the standard released in 1992, "SQL-99" refers to the standard released in 1999, and "SQL:2003" refers to the version of the standard that is expected to be released in mid-2003.We use the term "the SQL standard" to mean the current version of the SQL Standard at any time.)

MySQL software is Open Source.

Open Source means that it is possible for anyone to use and modify the software. Anybody can download the MySQL software from the Internet and use it without paying anything. If you wish, you may study the source code and change it to suit your needs. The MySQL software uses the GPL (GNU General Public License), http://www.gnu.org/licenses/, to define what you may and may not do with the software in different situations. If you feel uncomfortable with the GPL or need to embed MySQL code into a commercial application you can buy a commercially licensed version from us. See section MySQL Licenses.

Why use the MySQL Database Server?

The MySQL Database Server is very fast, reliable, and easy to use. If that is what you are looking for, you should give it a try. MySQL Server also has a practical set of features developed in close cooperation with our users. You can find a performance comparison of MySQL Server with other database managers on our benchmark page. See section The MySQL Benchmark Suite.

MySQL Server was originally developed to handle large databases much faster than existing solutions and has been successfully used in highly demanding production environments for several years. Though under constant development, MySQL Server today offers a rich and useful set of functions. Its connectivity, speed, and security make MySQL Server highly suited for accessing databases on the Internet.

The technical features of MySQL Server

For advanced technical information, see MySQL Language Reference. The MySQL Database Software is a client/server system that consists of a multi-threaded SQL server that supports different backends, several different client programs and libraries, administrative tools, and a wide range of programming interfaces (APIs).

We also provide MySQL Server as a multi-threaded library which you can link into your application to get a smaller, faster, easier-to-manage product.

There is a large amount of contributed MySQL software available.

It is very likely that you will find that your favorite application or language already supports the MySQL Database Server.

The official way to pronounce MySQL is "My Ess Que Ell" (not "my sequel"), but we don't mind if you pronounce it as "my sequel" or in some other localised way.

1.2.1 History of MySQL

We started out 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 nor 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 clear. Our base directory and a large number of our libraries and tools have had the prefix "my" for well over 10 years. However, co-founder Monty Widenius's daughter (some years younger) is also named My. Which of the two gave its name to MySQL is still a mystery, even for us.

The name of the MySQL Dolphin (our logo) is Sakila. Sakila was chosen by the founders of MySQL AB from a huge list of names suggested by users in our "Name the Dolphin" contest. The winning name was submitted by Ambrose Twebaze, an open source software developer from Swaziland, Africa. According to Ambrose, the name Sakila has its roots in SiSwati, the local language of Swaziland. Sakila is also the name of a town in Arusha, Tanzania, near Ambrose's country of origin, Uganda.

1.2.2 The Main Features of MySQL

The following list describes some of the important characteristics of the MySQL Database Software. See section MySQL 4.0 in a Nutshell.

Internals and Portability
Column Types
Commands and Functions
Scalability and Limits
Clients and Tools

1.2.3 How Stable Is MySQL?

This section addresses the questions "How stable is MySQL Server?" and "Can I depend on MySQL Server in this project?" We will try to clarify these issues and answer some important questions that concern many potential users. The information in this section is based on data gathered from the mailing list, which is very active in identifying problems as well as reporting types of use.

Original code stems back from the early '80s, providing a stable code base, and the ISAM table format remains backward-compatible. At TcX, the predecessor of MySQL AB, MySQL code has worked in projects since mid-1996, without any problems. When the MySQL Database Software was released to a wider public, our new users quickly found some pieces of "untested code". Each new release since then has had fewer portability problems (even though each new release has also had many new features).

Each release of the MySQL Server has been usable. Problems have occurred only when users try code from the "gray zones." Naturally, new users don't know what the gray zones are; this section therefore attempts to document those areas that are currently known. The descriptions mostly deal with Version 3.23 and 4.0 of MySQL Server. All known and reported bugs are fixed in the latest version, with the exception of those listed in the bugs section, which are things that are design-related. See section Known Errors and Design Deficiencies in MySQL.

The MySQL Server design is multi-layered with independent modules. Some of the newer modules are listed here with an indication of how well-tested each of them is:

Replication - Gamma

Large server clusters using replication are in production use, with good results. Work on enhanced replication features is continuing in MySQL 4.x.

InnoDB tables - Stable (in 3.23 from 3.23.49)

The InnoDB transactional storage engine has been declared stable in the MySQL 3.23 tree, starting from version 3.23.49. InnoDB is being used in large, heavy-load production systems.

BDB tables - Gamma

The Berkeley DB code is very stable, but we are still improving the BDB transactional storage engine interface in MySQL Server, so it will take some time before this is as well tested as the other table types.


Full-text search works but is not yet widely used. Important enhancements have been implemented in MySQL 4.0.

MyODBC 3.51 (uses ODBC SDK 3.51) - Stable

In wide production use. Some issues brought up appear to be application-related and independent of the ODBC driver or underlying database server.

Automatic recovery of MyISAM tables - Gamma

This status applies only to the new code in the MyISAM storage engine that checks if the table was closed properly on open and executes an automatic check/repair of the table if it wasn't.

Bulk-insert - Alpha

New feature in MyISAM tables in MySQL 4.0 for faster insert of many rows.

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 mysqld with the --skip-external-locking flag. Problems are known to occur on some Linux systems, and on SunOS when using NFS-mounted filesystems.

MySQL AB provides high-quality support for paying customers, and 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.

1.2.4 How Big Can MySQL Tables Be?

MySQL Version 3.22 had a 4 GB (4 gigabyte) limit on table size. With the MyISAM table type in MySQL Version 3.23, the maximum table size was pushed up to 8 million terabytes (2 ^ 63 bytes).

In effect, then, the table size for MySQL databases is normally limited by the operating system.

Note, however, that operating systems have their own file-size limits. Here are some examples:

Operating System

File-Size Limit

Linux-Intel 32 bit

2 GB, much more when using LFS


8 TB (?)

Solaris 2.5.1

2 GB (possible 4GB with patch)

Solaris 2.6

4 GB (can be changed with flag)

Solaris 2.7 Intel

4 GB

Solaris 2.7 UltraSPARC

512 GB

On Linux 2.2 you can get tables larger than 2 GB in size by using the LFS patch for the ext2 filesystem. On Linux 2.4 patches also exist for ReiserFS to get support for big files. Most current distributions are based on kernel 2.4 and already include all the required Large File Support (LFS) patches. However, the maximum available file size still depends on several factors, one of them being the file system used to store MySQL tables.

For a very detailed overview about LFS in Linux, have a look at Andreas Jaeger's "Large File Support in Linux" page at http://www.suse.de/~aj/linux_lfs.html.

By default, MySQL tables have a maximum size of about 4 GB. You can check the maximum table size for a table with the SHOW TABLE STATUS command or with the myisamchk -dv table_name. See section SHOW Syntax.

If you need a table that will be larger than 4 GB in size (and your operating system supports it), set the AVG_ROW_LENGTH and MAX_ROWS parameters accordingly when you create your table. See section CREATE TABLE Syntax. You can also set these parameters later, with ALTER TABLE. See section ALTER TABLE Syntax.

If your big table is a read-only table, you could use myisampack to merge and compress many tables into one. myisampack usually compresses a table by at least 50%, so you can have, in effect, much bigger tables. See section myisampack.

You can get around the operating system file limit for MyISAM datafiles using the RAID option. See section CREATE TABLE Syntax.

Another solution can be the included MERGE library, which allows you to handle a collection of identical tables as one. See section MERGE tables.

1.2.5 Year 2000 Compliance

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

You may run into problems with applications that use MySQL Server 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 because 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 Server doesn't have any problems with dates until the year 2030:

Query OK, 0 rows affected (0.01 sec)

mysql> CREATE TABLE y2k (date DATE,
    ->                   date_time DATETIME,
    ->                   time_stamp TIMESTAMP);
Query OK, 0 rows affected (0.00 sec)

    -> ("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",20500101000000);
Query OK, 13 rows affected (0.01 sec)
Records: 13  Duplicates: 0  Warnings: 0

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)

This example shows that the DATE and DATETIME datatypes will not give any problems with future dates (they handle dates until the year 9999).

The TIMESTAMP datatype, which is used to store the current time, supports values that range from 19700101000000 to 20300101000000 on 32-bit machines (signed value). On 64-bit machines, TIMESTAMP handles values up to 2106 (unsigned value).

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

1.3 What Is MySQL AB?

MySQL AB is the company of the MySQL founders and main developers. MySQL AB was originally established in Sweden by David Axmark, Allan Larsson, and Michael Monty Widenius.

The developers of the MySQL server are all employed by the company. We are a virtual organisation with people in a dozen countries around the world. We communicate extensively over the Net every day with one another and with our users, supporters, and partners.

We are dedicated to developing the MySQL software and spreading our database to new users. MySQL AB owns the copyright to the MySQL source code, the MySQL logo and trademark, and this manual. See section What Is MySQL?.

The MySQL core values show our dedication to MySQL and Open Source.

We want the MySQL Database Software to be:

MySQL AB and the people at MySQL AB:

The MySQL web site (http://www.mysql.com/) provides the latest information about MySQL and MySQL AB.

By the way, the "AB" part of the company name is the acronym for the Swedish "aktiebolag", or "stock company." It translates to "MySQL, Inc." In fact, MySQL Inc. and MySQL GmbH are examples of MySQL AB subsidiaries. They are located in the US and Germany, respectively.

1.3.1 The Business Model and Services of MySQL AB

One of the most common questions we encounter is: "How can you make a living from something you give away for free?" This is how.

MySQL AB makes money on support, services, commercial licenses, and royalties. We use these revenues to fund product development and to expand the MySQL business.

The company has been profitable since its inception. In October 2001, we accepted venture financing from leading Scandinavian investors and a handful of business angels. This investment is used to solidify our business model and build a basis for sustainable growth. Support

MySQL AB is run and owned by the founders and main developers of the MySQL database. The developers are committed to giving support to customers and other users in order to stay in touch with their needs and problems. All our support is given by qualified developers. Really tricky questions are answered by Michael Monty Widenius, principal author of the MySQL Server. See section Support Offered by MySQL AB.

For more information and ordering support at various levels, see http://www.mysql.com/support/ or contact our sales staff at sales@mysql.com. Training and Certification

MySQL AB delivers MySQL and related training worldwide. We offer both open courses and in-house courses tailored to the specific needs of your company. MySQL Training is also available through our partners, the Authorised MySQL Training Centers.

Our training material uses the same example databases used in our documentation and our sample applications, and is always updated to reflect the latest MySQL version. Our trainers are backed by the development team to guarantee the quality of the training and the continuous development of the course material. This also ensures that no questions raised during the courses remain unanswered.

Attending our training courses will enable you to achieve your MySQL application goals. You will also:

If you are interested in our training as a potential participant or as a training partner, please visit the training section at http://www.mysql.com/training/ or contact us at: training@mysql.com.

For details about the MySQL Certification Program, please see http://www.mysql.com/certification/. Consulting

MySQL AB and its Authorised Partners offer consulting services to users of MySQL Server and to those who embed MySQL Server in their own software, all over the world.

Our consultants can help you design and tune your databases, construct efficient queries, tune your platform for optimal performance, resolve migration issues, set up replication, build robust transactional applications, and more. We also help customers embed MySQL Server in their products and applications for large-scale deployment.

Our consultants work in close collaboration with our development team, which ensures the technical quality of our professional services. Consulting assignments range from 2-day power-start sessions to projects that span weeks and months. Our expertise not only covers MySQL Server--it also extends into programming and scripting languages such as PHP, Perl, and more.

If you are interested in our consulting services or want to become a consulting partner, please visit the consulting section of our web site at http://www.mysql.com/consulting/ or contact our consulting staff at consulting@mysql.com. Commercial Licenses

The MySQL database is released under the GNU General Public License (GPL). This means that the MySQL software can be used free of charge under the GPL. If you do not want to be bound by the GPL terms (such as the requirement that your application must also be GPL, you may purchase a commercial license for the same product from MySQL AB; see http://www.mysql.com/products/pricing.html. Since MySQL AB owns the copyright to the MySQL source code, we are able to employ Dual Licensing, which means that the same product is available under GPL and under a commercial license. This does not in any way affect the Open Source commitment of MySQL AB. For details about when a commercial license is required, please see MySQL Licenses.

We also sell commercial licenses of third-party Open Source GPL software that adds value to MySQL Server. A good example is the InnoDB transactional storage engine that offers ACID support, row-level locking, crash recovery, multi-versioning, foreign key support, and more. See section InnoDB Tables. Partnering

MySQL AB has a worldwide partner programme that covers training courses, consulting and support, publications, plus reselling and distributing MySQL and related products. MySQL AB Partners get visibility on the http://www.mysql.com/ web site and the right to use special versions of the MySQL trademarks to identify their products and promote their business.

If you are interested in becoming a MySQL AB Partner, please e-mail partner@mysql.com.

The word MySQL and the MySQL dolphin logo are trademarks of MySQL AB. See section MySQL AB Logos and Trademarks. These trademarks represent a significant value that the MySQL founders have built over the years.

The MySQL web site (http://www.mysql.com/) is popular among developers and users. In October 2001, we served 10 million page views. Our visitors represent a group that makes purchase decisions and recommendations for both software and hardware. Twelve percent of our visitors authorise purchase decisions, and only nine percent are not involved in purchase decisions at all. More than 65% have made one or more online business purchases within the last half-year, and 70% plan to make one in the next few months.

1.3.2 Contact Information

The MySQL web site (http://www.mysql.com/) provides the latest information about MySQL and MySQL AB.

For press services and inquiries not covered in our News releases (http://www.mysql.com/news/), please send an e-mail to press@mysql.com.

If you have a valid support contract with MySQL AB, you will get timely, precise answers to your technical questions about the MySQL software. For more information, see Support Offered by MySQL AB. On our web site, see http://www.mysql.com/support/, or send an e-mail to sales@mysql.com.

For information about MySQL training, please visit the training section at http://www.mysql.com/training/. If you have restricted access to the Internet, please contact the MySQL AB training staff via e-mail at training@mysql.com. See section Training and Certification.

For information on the MySQL Certification Program, please see http://www.mysql.com/certification/. See section Training and Certification.

If you're interested in consulting, please visit the consulting section of our web site at http://www.mysql.com/consulting/. If you have restricted access to the Internet, please contact the MySQL AB consulting staff via e-mail at consulting@mysql.com. See section Consulting.

Commercial licenses may be purchased online at https://order.mysql.com/. There you will also find information on how to fax your purchase order to MySQL AB. More information about licensing can be found at http://www.mysql.com/products/pricing.html. If you have questions regarding licensing or you want a quote for a high-volume license deal, please fill in the contact form on our web site (http://www.mysql.com/) or send an e-mail message to licensing@mysql.com (for licensing questions) or to sales@mysql.com (for sales inquiries). See section MySQL Licenses.

If you represent a business that is interested in partnering with MySQL AB, please send an e-mail to partner@mysql.com. See section Partnering.

For more information on the MySQL trademark policy, refer to http://www.mysql.com/company/trademark.html or send an e-mail to trademark@mysql.com. See section MySQL AB Logos and Trademarks.

If you are interested in any of the MySQL AB jobs listed in our jobs section (http://www.mysql.com/company/jobs/), please send an e-mail to jobs@mysql.com. Please do not send your CV as an attachment, but rather as plain text at the end of your e-mail message.

For general discussion among our many users, please direct your attention to the appropriate mailing list. See section MySQL Mailing Lists.

Reports of errors (often called bugs), as well as questions and comments, should be sent to the general MySQL mailing list. See section The MySQL Mailing Lists. If you have found a sensitive security bug in the MySQL Server, please send an e-mail to security@mysql.com. See section How to Report Bugs or Problems.

If you have benchmark results that we can publish, please contact us via e-mail at benchmarks@mysql.com.

If you have suggestions concerning additions or corrections to this manual, please send them to the manual team via e-mail at docs@mysql.com.

For questions or comments about the workings or content of the MySQL web site (http://www.mysql.com/), please send an e-mail to webmaster@mysql.com.

MySQL AB has a privacy policy, which can be read at http://www.mysql.com/company/privacy.html. For any queries regarding this policy, please send an e-mail to privacy@mysql.com.

For all other inquires, please send an e-mail to info@mysql.com.

1.4 MySQL Support and Licensing

This section describes MySQL support and licensing arrangements.

1.4.1 Support Offered by MySQL AB

Technical support from MySQL AB means individualised answers to your unique problems direct from the software engineers who code the MySQL database engine.

We try to take a broad and inclusive view of technical support. Almost any problem involving MySQL software is important to us if it's important to you. Typically customers seek help on how to get different commands and utilities to work, remove performance bottlenecks, restore crashed systems, understand operating system or networking impacts on MySQL, set up best practices for backup and recovery, utilise APIs, and so on. Our support covers only the MySQL server and our own utilities, not third-party products that access the MySQL server, though we try to help with these where we can.

Detailed information about our various support options is given at http://www.mysql.com/support/, where support contracts can also be ordered online. If you have restricted access to the Internet, please contact our sales staff via e-mail at sales@mysql.com.

Technical support is like life insurance. You can live happily without it for years, but when your hour arrives it becomes critically important, yet it's too late to buy it. If you use MySQL Server for important applications and encounter sudden difficulties, it may be too time consuming to figure out all the answers yourself. You may need immediate access to the most experienced MySQL troubleshooters available, those employed by MySQL AB.

1.4.2 Copyrights and Licenses Used by MySQL

MySQL AB owns the copyright to the MySQL source code, the MySQL logos and trademarks and this manual. See section What Is MySQL AB?. Several different licenses are relevant to the MySQL distribution:

  1. All the MySQL-specific source in the server, the mysqlclient library and the client, as well as the GNU readline library is covered by the GNU General Public License. See section GNU General Public License. The text of this license can be found as the file `COPYING' in the distribution.
  2. The GNU getopt library is covered by the GNU Lesser General Public License. See section GNU Lesser General Public License.
  3. Some parts of the source (the regexp library) are covered by a Berkeley-style copyright.
  4. Older versions of MySQL (3.22 and earlier) are subject to a stricter license (http://www.mysql.com/products/mypl.html). See the documentation of the specific version for information.
  5. The MySQL reference manual is currently not distributed under a GPL-style license. Use of the manual is subject to the following terms:

    Please send an e-mail to docs@mysql.com for more information or if you are interested in doing a translation.

For information about how the MySQL licenses work in practice, please refer to MySQL Licenses. Also see MySQL AB Logos and Trademarks.

1.4.3 MySQL Licenses

The MySQL software is released under the GNU General Public License (GPL), which is probably the best known Open Source license. The formal terms of the GPL license can be found at http://www.gnu.org/licenses/. See also http://www.gnu.org/licenses/gpl-faq.html and http://www.gnu.org/philosophy/enforcing-gpl.html.

Since the MySQL software is released under the GPL, it may often be used for free, but for certain uses you may want or need to buy commercial licenses from MySQL AB at https://order.mysql.com/. See http://www.mysql.com/products/licensing.html for more information.

Older versions of MySQL (3.22 and earlier) are subject to a stricter license (http://www.mysql.com/products/mypl.html). See the documentation of the specific version for information.

Please note that the use of the MySQL software under commercial license, GPL, or the old MySQL license does not automatically give you the right to use MySQL AB trademarks. See section MySQL AB Logos and Trademarks. Using the MySQL Software Under a Commercial License

The GPL license is contagious in the sense that when a program is linked to a GPL program all the source code for all the parts of the resulting product must also be released under the GPL. If you do not follow this GPL requirement, you break the license terms and forfeit your right to use the GPL program altogether. You also risk damages.

You need a commercial license:

If you require a license, you will need one for each installation of the MySQL software. This covers any number of CPUs on a machine, and there is no artificial limit on the number of clients that connect to the server in any way.

For commercial licenses, please visit our website at http://www.mysql.com/products/licensing.html. For support contracts, see http://www.mysql.com/support/. If you have special needs or you have restricted access to the Internet, please contact our sales staff via e-mail at sales@mysql.com. Using the MySQL Software for Free Under GPL

You can use the MySQL software for free under the GPL if you adhere to the conditions of the GPL. For additional details, including answers to common questions about the GPL, see the generic FAQ from the Free Software Foundation at http://www.gnu.org/licenses/gpl-faq.html. Common uses of the GPL include:

If your use of MySQL database software does not require a commercial license, we encourage you to purchase support from MySQL AB anyway. This way you contribute toward MySQL development and also gain immediate advantages for yourself. See section Support Offered by MySQL AB.

If you use the MySQL database software in a commercial context such that you profit by its use, we ask that you further the development of the MySQL software by purchasing some level of support. We feel that if the MySQL database helps your business, it is reasonable to ask that you help MySQL AB. (Otherwise, if you ask us support questions, you are not only using for free something into which we've put a lot a work, you're asking us to provide free support, too.)

1.4.4 MySQL AB Logos and Trademarks

Many users of the MySQL database want to display the MySQL AB dolphin logo on their web sites, books, or boxed products. We welcome and encourage this, although it should be noted that the word MySQL and the MySQL dolphin logo are trademarks of MySQL AB and may only be used as stated in our trademark policy at http://www.mysql.com/company/trademark.html. The Original MySQL Logo

The MySQL dolphin logo was designed by the Finnish advertising agency Priority in 2001. The dolphin was chosen as a suitable symbol for the MySQL database since it is a smart, fast, and lean animal, effortlessly navigating oceans of data. We also happen to like dolphins.

The original MySQL logo may only be used by representatives of MySQL AB and by those having a written agreement allowing them to do so. MySQL Logos that may be Used Without Written Permission

We have designed a set of special Conditional Use logos that may be downloaded from our web site at http://www.mysql.com/press/logos.html and used on third-party web sites without written permission from MySQL AB. The use of these logos is not entirely unrestricted but, as the name implies, subject to our trademark policy that is also available on our web site. You should read through the trademark policy if you plan to use them. The requirements are basically as follows:

Contact us via e-mail at trademark@mysql.com to inquire about special arrangements to fit your needs. When do you need a Written Permission to use MySQL Logos?

You need written permission from MySQL AB before using MySQL logos in the following cases:

Due to legal and commercial reasons we monitor the use of MySQL trademarks on products, books, and other items. We usually require a fee for displaying MySQL AB logos on commercial products, since we think it is reasonable that some of the revenue is returned to fund further development of the MySQL database. MySQL AB Partnership Logos

MySQL partnership logos may be used only by companies and persons having a written partnership agreement with MySQL AB. Partnerships include certification as a MySQL trainer or consultant. For more information, please see Partnering. Using the word MySQL in Printed Text or Presentations

MySQL AB welcomes references to the MySQL database, but it should be noted that the word MySQL is a trademark of MySQL AB. Because of this, you must append the trademark symbol (TM) to the first or most prominent use of the word MySQL in a text and, where appropriate, state that MySQL is a trademark of MySQL AB. For more information, please refer to our trademark policy at http://www.mysql.com/company/trademark.html. Using the word MySQL in Company and Product Names

Use of the word MySQL in product or company names or in Internet domain names is not allowed without written permission from MySQL AB.

1.5 MySQL Development Roadmap

This section provides a snapshot of the MySQL development roadmap, including major features implemented or planned for MySQL 4.0, 4.1, and 5.0. The following sections provide information for each release. Plans for some of the most requested features are summarized in the following table.


MySQL version






4.1 (for MyISAM tables)

Stored procedures




Foreign keys

5.1 (3.23 with InnoDB)





Full outer join




1.5.1 MySQL 4.0 in a Nutshell

Long promised by MySQL AB and long awaited by our users, MySQL Server 4.0 is now available in production version.

MySQL 4.0 is available for download from http://www.mysql.com/ and from our mirrors. MySQL 4.0 has been tested by a large number of users and is in production use at many large sites.

The major new features of MySQL Server 4.0 are geared toward our existing business and community users, enhancing the MySQL database software as the solution for mission-critical, heavy-load database systems. Other new features target the users of embedded databases.

MySQL Version 4.0.12 was declared stable for production use in March 2003. This means that, in future, only bug fixes will be done for the 4.0 release series and only critical bug fixes will be done for the older 3.23 series. See section Upgrading From Version 3.23 to 4.0.

New features to the MySQL software are being added to MySQL 4.1 which is now also available (alpha version). See section MySQL 4.1 in a Nutshell. Features Available in MySQL 4.0

Speed enhancements
Embedded MySQL Server introduced
InnoDB storage engine as standard
New functionality
Standards compliance, portability, and migration
Usability enhancements

In the process of building features for new users, we have not forgotten requests from our community of loyal users.

The news section of this manual includes a more in-depth list of features. See section Changes in release 4.0.x (Production). Embedded MySQL Server

libmysqld makes MySQL Server suitable for a vastly expanded realm of applications. Using the embedded MySQL server library, one can embed MySQL Server into various applications and electronics devices, where the end user has no knowledge of there actually being an underlying database. Embedded MySQL Server is ideal for use behind the scenes in Internet appliances, public kiosks, turnkey hardware/software combination units, high performance Internet servers, self-contained databases distributed on CD-ROM, and so on.

Many users of libmysqld will benefit from the MySQL Dual Licensing. For those not wishing to be bound by the GPL, the software is also made available under a commercial license. The embedded MySQL library uses the same interface as the normal client library, so it is convenient and easy to use. See section libmysqld, the Embedded MySQL Server Library.

1.5.2 MySQL 4.1 in a Nutshell

MySQL Server 4.0 laid the foundation for new features such as nested subqueries and Unicode (implemented in version 4.1) and for the work on SQL-99 stored procedures being done for version 5.0. These features come at the top of the wish list of many of our customers.

With these additions, critics of the MySQL Database Server have to be more imaginative than ever in pointing out deficiencies in the MySQL Database Management System. Already well-known for its stability, speed, and ease of use, MySQL Server will be able to fulfill the requirement checklists of very demanding buyers. Features Available in MySQL 4.1

The features listed in this section are implemented in MySQL 4.1. A few other features are still planned for MySQL 4.1. See section New Features Planned For 4.1.

Most new features being coded, such as stored procedures, will be available in MySQL 5.0. See section New Features Planned For 5.0.

Support for subqueries and derived tables
Speed enhancements
New functionality
Standards compliance, portability, and migration
Usability enhancements

The news section in this manual includes a more in-depth list of features. See section Changes in release 4.1.x (Alpha). Stepwise Rollout

New features are being added to MySQL 4.1, which is already available for download (alpha version). See section Ready for Immediate Development Use.

The set of features that are being added to version 4.1 is mostly fixed. Additional development is already ongoing for version 5.0. MySQL 4.1 will go through the steps of Alpha (during which time new features might still be added/changed), Beta (when we have feature freeze and only bug corrections will be done), and Gamma (indicating that a production release is just weeks ahead). At the end of this process, MySQL 4.1 will become the new production release. Ready for Immediate Development Use

MySQL 4.1 is currently in the alpha stage, and binaries are available for download at http://www.mysql.com/downloads/mysql-4.1.html. All binary releases pass our extensive test suite without any errors on the platforms on which we test. See section Changes in release 4.1.x (Alpha).

For those wishing to use the most recent development source for MySQL 4.1, we have made our 4.1 BitKeeper repository publicly available. See section Installing from the Development Source Tree.

1.5.3 MySQL 5.0, The Next Development Release

New development for MySQL is focused on the 5.0 release, featuring Stored Procedures and other new features. See section New Features Planned For 5.0.

For those wishing to take a look at the bleeding edge of MySQL development, we have made our BitKeeper repository for MySQL version 5.0 publicly available. See section Installing from the Development Source Tree.

1.6 MySQL Information Sources

1.6.1 MySQL Mailing Lists

This section introduces you to the MySQL mailing lists and gives some guidelines as to how the lists should be used. When you subscribe to a mailing list, you will receive, as e-mail messages, all postings to the list. You will also be able to send your own questions and answers to the list. The MySQL Mailing Lists

To subscribe to or unsubscribe from any of the mailing lists described in this section, visit http://lists.mysql.com/. Please do not send messages about subscribing or unsubscribing to any of the mailing lists, because such messages are distributed automatically to thousands of other users.

Your local site may have many subscribers to a MySQL mailing list. If so, the site 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.

If you wish to have traffic for a mailing list go to a separate mailbox in your mail program, set up a filter based on the message headers. You can use either the List-ID: or Delivered-To: headers to identify list messages.

The MySQL mailing lists are as follows:


This list is for announcements of new versions of MySQL and related programs. This is a low-volume list to which all MySQL users should subscribe.


This is the main list for general MySQL discussion. Please note that some topics are better discussed on the more-specialised lists. If you post to the wrong list, you may not get an answer.


This is the mysql list in digest form. Subscribing to this list means you will get all list messages, sent as one large mail message once a day.


This list will be of interest to you if you want to stay informed about issues reported since the last release of MySQL or if you want to be actively involved in the process of bug hunting and fixing. See section How to Report Bugs or Problems.


This is the bugs list in digest form.


This list is for people who work on the MySQL code. This is also the forum for discussions on MySQL development and post patches.


This is the internals list in digest form.


This list is for people who work on the MySQL documentation: people from MySQL AB, translators, and other community members.


This is the mysqldoc list in digest form.


This list is for anyone interested in performance issues. Discussions concentrate on database performance (not limited to MySQL) but also include broader categories such as performance of the kernel, file system, disk system, and so on.


This is the benchmarks list in digest form.


This list is for discussions on packaging and distributing MySQL. This is the forum used by distribution maintainers to exchange ideas on packaging MySQL and on ensuring that MySQL looks and feels as similar as possible on all supported platforms and operating systems.


This is the packagers list in digest form.


This list is for discussions about the MySQL server and Java.It is mostly used to discuss JDBC drivers, including MySQL Connector/J.


This is the java list in digest form.


This list is for all things concerning the MySQL software on Microsoft operating systems, such as Windows 9x/Me/NT/2000/XP.


This is the win32 list in digest form.


This list is for all things concerning connecting to the MySQL server with ODBC.


This is the myodbc list in digest form.


This list is for all things concerning the MySQL Control Center graphical client.


This is the mysqlcc list in digest form.


This list is for all things concerning programming with the C++ API to MySQL.


This is the plusplus list in digest form.


This list is for all things concerning the Perl support for MySQL with msql-mysql-modules, which is now named DBD-mysql.


This is the msql-mysql-modules list in digest form.

If you're unable to get an answer to your question(s) from a MySQL mailing list, one option is to pay for support from MySQL AB. This will put you in direct contact with MySQL developers. See section Support Offered by MySQL AB.

The following table shows some MySQL mailing lists in languages other than English. These lists are not operated by MySQL AB, so we can't guarantee their quality.

mysql-france-subscribe@yahoogroups.com A French mailing list
list@tinc.net A Korean mailing list

E-mail subscribe mysql your@e-mail.address to this list.

mysql-de-request@lists.4t2.com A German mailing list

E-mail subscribe mysql-de your@e-mail.address to this list. You can find information about this mailing list at http://www.4t2.com/mysql/.

mysql-br-request@listas.linkway.com.br A Portuguese mailing list

E-mail subscribe mysql-br your@e-mail.address to this list.

mysql-alta@elistas.net A Spanish mailing list

E-mail subscribe mysql your@e-mail.address to this list. 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, please follow the guidelines on sending mail to a MySQL mailing list, outlined in the next section, before contacting us. How to Report Bugs or Problems

Our bugs database is public, and can be browsed and searched by anyone at http://bugs.mysql.com/. If you log into the system, you will also be able to enter new reports.

Writing a good bug report takes patience, but doing it right the first time saves time both for us and for yourself. A good bug report, containing a full test case for the bug, makes it very likely that we will fix the bug in the next release. 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). mysqlbug can be found in the `scripts' directory (source distribution) and in the `bin' directory under your MySQL installation directory (binary distribution). If you are unable to use mysqlbug (for instance, if you are running on Windows), it is still vital that you include all the necessary information noted in this section (most importantly a description of the operating system and the MySQL version).

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.

Preferably, you should test the problem using the latest production or development version of MySQL Server before posting. Anyone should be able to repeat the bug by just using 'mysql test < script' on the included test case or run the shell or Perl script that is included in the bug report.

All bugs posted in the bugs database at http://bugs.mysql.com/ will be corrected or documented in the next MySQL release. If only minor code changes are needed to correct a problem, we will also post a patch that fixes the problem.

The normal place to report bugs is http://bugs.mysql.com/.

If you have found a sensitive security bug in MySQL, please send an e-mail to security@mysql.com.

If you have a repeatable bug report, please report this into the bugs database at http://bugs.mysql.com/. Note that even in this case it's good to run the mysqlbug script first to find information about your system. Any bug that we are able to repeat has a high chance of being fixed in the next MySQL release.

To report other problems, you can use one of the MySQL mailing lists.

Remember that it is possible for us to respond to a message containing too much information, but not to one containing too little. People often 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 made in bug reports are (a) not including the version number of the MySQL distribution used and (b) not fully describing the platform on which the MySQL server is installed (including the platform type and 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?" Then we find that the feature requested wasn't implemented in that MySQL version, or that a bug described in a report has already been fixed in newer MySQL versions. Sometimes the error is platform-dependent; in such cases, it is next to impossible for us 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. To determine whether your problem depends on your compiler, we need to know what compiler you use. Note that every compiling problem should be regarded as a bug and reported accordingly.

It is most helpful when a good description of the problem is included in the bug report. That is, give a good example of all the things you did that led to the problem and describe, in exact detail, the problem itself. The best reports are those that include a full example showing how to reproduce the bug or problem. See section Making a Test Case If You Experience Table Corruption.

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 should be observed.) You should never try to remember what the error message was; instead, copy and paste the entire message into your report.

If you have a problem with MyODBC, please try to generate a MyODBC trace file and send it with your report. See section Reporting Problems with MyODBC.

Please remember that many of the people who will read your report will do so using an 80-column display. When generating reports or examples using the mysql command-line tool, you should therefore use the --vertical option (or the \G statement terminator) for output that would exceed the available width for such a display (for example, with the EXPLAIN SELECT statement; see the example later in this section).

Please include the following information in your report:

If you are a support customer, please cross-post the bug report to mysql-support@mysql.com for higher-priority treatment, as well as 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 How to Report Problems with MyODBC.

For solutions to some common problems, see Problems and Common Errors.

When answers are sent to you individually and not to the mailing list, it is considered good etiquette to summarise 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. 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 summarise 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 don't read mail with a browser.

1.6.2 MySQL Community Support on IRC (Internet Relay Chat)

In addition to the various MySQL mailing lists, you can find experienced community people on IRC (Internet Relay Chat). These are the best networks/channels currently known to us:

If you are looking for IRC client software to connect to an IRC network, take a look at X-Chat (http://www.xchat.org/). X-Chat (GPL licensed) is available for Unix as well as for Windows platforms.

1.7 MySQL Standards Compliancy

This section describes how MySQL relates to the ANSI/ISO SQL standards. MySQL Server has many extensions to the SQL standard, and here you will find out what they are and how to use them. You will also find information about functionality missing from MySQL Server, and how to work around some differences.

Our goal is to not, without a very good reason, restrict MySQL Server usability for any usage. Even if we don't have the resources to do development for every possible use, we are always willing to help and offer suggestions to people who are trying to use MySQL Server in new territories.

One of our main goals with the product is to continue to work toward compliance with the SQL-99 standard, but without sacrificing speed or reliability. We are not afraid to add extensions to SQL or support for non-SQL features if this greatly increases the usability of MySQL Server for a big part of our users. (The new HANDLER interface in MySQL Server 4.0 is an example of this strategy. See section HANDLER.)

We will continue to support transactional and non-transactional databases to satisfy both heavy web/logging usage and mission-critical 24/7 usage.

MySQL Server was designed from the start to work with medium size databases (10-100 million rows, or about 100 MB per table) on small computer systems. We will continue to extend MySQL Server to work even better with terabyte-size databases, as well as to make it possible to compile a reduced MySQL version that is more suitable for hand-held devices and embedded usage. The compact design of the MySQL server makes both of these directions possible without any conflicts in the source tree.

We are currently not targeting realtime support or clustered databases (even if you can already do a lot of things with our replication services).

We are looking at providing XML support in the database server.

1.7.1 What Standards Does MySQL Follow?

Entry-level SQL-92. ODBC levels 0-3.51.

We are aiming toward supporting the full SQL-99 standard, but without concessions to speed and quality of the code.

1.7.2 Running MySQL in ANSI Mode

If you start mysqld with the --ansi or --sql-mode=ANSI option, the following behaviours of MySQL Server change:

Running the server in ANSI mode is the same as starting it with these options:


In MySQL 4.1, you can achieve the same effect with these two statements:

SET GLOBAL sql_mode =

In MySQL 4.1.1, the sql_mode options shown can be also be set with:

SET GLOBAL sql_mode="ansi";

In this case, the value of the sql_mode variable will be set to all options that are relevant for ANSI mode. You can check the result by doing:

mysql> SET GLOBAL sql_mode="ansi";
mysql> SELECT @@GLOBAL.sql_mode;

1.7.3 MySQL Extensions To The SQL-92 Standard

MySQL Server 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 Server 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 be executed only if the MySQL version is equal to or newer than the used version number:


This means that if you have Version 3.23.02 or newer, MySQL Server will use the TEMPORARY keyword.

The following is a list of MySQL extensions:

1.7.4 MySQL Differences Compared To SQL-92

We try to make MySQL Server follow the ANSI SQL standard (SQL-92/SQL-99) and the ODBC SQL standard, but in some cases MySQL Server does things differently:

For a prioritised list indicating when new extensions will be added to MySQL Server, you should consult the online MySQL TODO list at http://www.mysql.com/doc/en/TODO.html. That is the latest version of the TODO list in this manual. See section MySQL and The Future (The TODO). Subqueries

Subqueries are supported in MySQL version 4.1. See section Features Available in MySQL 4.1.

Up to version 4.0, only nested queries of the form INSERT ... SELECT ... and REPLACE ... SELECT ... are supported. You can, however, use the function IN() in other contexts.

You can often rewrite the query without a subquery:

SELECT * FROM table1 WHERE id IN (SELECT id FROM table2);

This can be rewritten as:

SELECT table1.* FROM table1,table2 WHERE table1.id=table2.id;

The queries:

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

Can be rewritten as:

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

Using a LEFT [OUTER] JOIN is generally much faster than an equivalent subquery because the server can optimise it better, a fact that is not specific to MySQL Server alone. Prior to SQL-92, outer joins did not exist, so subqueries were the only way to do certain things in those bygone days. But that is no longer the case, MySQL Server and many other modern database systems offer a whole range of outer joins types.

For more complicated subqueries you can often create temporary tables to hold the subquery. In some cases, however, this option will not work. The most frequently encountered of these cases arises with DELETE statements, for which standard SQL does not support joins (except in subqueries). For this situation there are three options available:

MySQL Server 4.0 supports multi-table DELETEs that can be used to efficiently delete rows based on information from one table or even from many tables at the same time. SELECT INTO TABLE

MySQL Server doesn't yet support the Oracle SQL extension: SELECT ... INTO TABLE .... Instead, MySQL Server supports the SQL-99 syntax INSERT INTO ... SELECT ..., which is basically the same thing. See section INSERT ... SELECT Syntax.

INSERT INTO tblTemp2 (fldID) SELECT tblTemp1.fldOrder_ID
       FROM tblTemp1 WHERE tblTemp1.fldOrder_ID > 100;

Alternatively, you can use SELECT INTO OUTFILE... or CREATE TABLE ... SELECT. Transactions and Atomic Operations

MySQL Server (version 3.23-max and all versions 4.0 and above) supports transactions with the InnoDB and BDB Transactional storage engines. InnoDB provides full ACID compliance. See section MySQL Table Types.

The other non-transactional table types (such as MyISAM) in MySQL Server follow a different paradigm for data integrity called "Atomic Operations." In transactional terms, MyISAM tables effectively always operate in AUTOCOMMIT=1 mode. Atomic operations often offer comparable integrity with higher performance.

With MySQL Server supporting both paradigms, the user is able to decide if he needs the speed of atomic operations or if he needs to use transactional features in his applications. This choice can be made on a per-table basis.

As noted, the trade off for transactional vs. non-transactional table types lies mostly in performance. Transactional tables have significantly higher memory and diskspace requirements, and more CPU overhead. That said, transactional table types such as InnoDB do of course offer many unique features. MySQL Server's modular design allows the concurrent use of all these different storage engines to suit different requirements and deliver optimum performance in all situations.

But how does one use the features of MySQL Server to maintain rigorous integrity even with the non-transactional MyISAM tables, and how do these features compare with the transactional table types?

  1. In the transactional paradigm, if your applications are written in a way that is dependent on the calling of ROLLBACK instead of COMMIT in critical situations, transactions are more convenient. Transactions also ensure that unfinished updates or corrupting activities are not committed to the database; the server is given the opportunity to do an automatic rollback and your database is saved.

    MySQL Server, in almost all cases, allows you to resolve potential problems by including simple checks before updates and by running simple scripts that check the databases for inconsistencies and automatically repair or warn if such an inconsistency occurs. Note that just by using the MySQL log or even adding one extra log, one can normally fix tables perfectly with no data integrity loss.

  2. More often than not, critical transactional updates can be rewritten to be atomic. Generally speaking, all integrity problems that transactions solve can be done with LOCK TABLES or atomic updates, ensuring that you never will get an automatic abort from the server, which is a common problem with transactional database systems.
  3. Even a transactional system can lose data if the server goes down. The difference between different systems lies in just how small the time-lap is where they could lose data. No system is 100% secure, only "secure enough." Even Oracle, reputed to be the safest of transactional database systems, is reported to sometimes lose data in such situations.

    To be safe with MySQL Server, whether using transactional tables or not, you only need to have backups and have the binary logging turned on. With this you can recover from any situation that you could with any other transactional database system. It is, of course, always good to have backups, independent of which database system you use.

The transactional paradigm has its benefits and its drawbacks. Many users and application developers depend on the ease with which they can code around problems where an abort appears to be, or is necessary. However, even if you are new to the atomic operations paradigm, or more familiar with transactions, do consider the speed benefit that non-transactional tables can offer on the order of three to five times the speed of the fastest and most optimally tuned transactional tables.

In situations where integrity is of highest importance, MySQL Server offers transaction-level reliability and integrity even for non-transactional tables. If you lock tables with LOCK TABLES, all updates will stall until any integrity checks are made. If you only obtain a read lock (as opposed to a write lock), reads and inserts are still allowed to happen. The new inserted records will not be seen by any of the clients that have a read lock until they release their read locks. With INSERT DELAYED you can queue inserts into a local queue, until the locks are released, without having the client wait for the insert to complete. See section INSERT DELAYED Syntax.

"Atomic," in the sense that we mean it, is nothing magical. It only means that you can be sure that while each specific update is running, no other user can interfere with it, and there will never be an automatic rollback (which can happen with transactional tables if you are not very careful). MySQL Server also guarantees that there will not be any dirty reads.

Following are some techniques for working with non-transactional tables: Stored Procedures and Triggers

Stored procedures are being implemented in our version 5.0 development tree. See section Installing from the Development Source Tree.

This effort is based on SQL-99, which has a basic syntax similar (but not identical) to Oracle PL/SQL. In addition to this, we are implementing the SQL-99 framework to hook in external languages.

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 re-issuing the entire query but can refer to the stored procedure. This provides better overall 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. However, stored procedures of course do increase the load on the database server system, as more of the work is done on the server side and less on the client (application) side.

Triggers will also be implemented. A trigger is effectively a type of stored procedure, one 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 transactional table and that stored procedure automatically deletes the corresponding customer from a customer table when all his transactions are deleted. Foreign Keys

In MySQL Server 3.23.44 and up, InnoDB tables support checking of foreign key constraints, including CASCADE, ON DELETE, and ON UPDATE. See section FOREIGN KEY Constraints.

For other table types, MySQL Server only parses the FOREIGN KEY syntax in CREATE TABLE commands, but does not use/store this info.

Note that foreign keys in SQL are not used to join tables, but are used mostly for checking referential integrity (foreign key constraints). 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 JOIN. See section Using Foreign Keys.

When used as a constraint, FOREIGN KEYs don't need to be used if the application inserts rows into MyISAM tables in the proper order.

For MyISAM tables, you can work around the lack of ON DELETE 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 MySQL Server 4.0 you can use multi-table delete to delete rows from many tables with one command. See section DELETE Syntax.

The FOREIGN KEY syntax without ON DELETE ... is often used by ODBC applications to produce automatic WHERE clauses.

In the near future we will extend the FOREIGN KEY implementation so that the information is stored in the table specification file and may be retrieved by mysqldump and ODBC. At a later stage we will implement foreign key constraints for MyISAM tables as well.

Do keep in mind that foreign keys are often misused, which can cause severe problems. Even when used properly, it is not a magic solution for the referential integrity problem, although it can make things easier.

Some advantages of foreign key enforcement:

Disadvantages: Views

We plan to implement views in MySQL Server in version 5.1

Historically, MySQL Server has been most used in applications and on web systems where the application writer has full control over database usage. Of course, usage has shifted over time, and so we find that an increasing number of users now regard views as an important aspect.

Views are useful for allowing users to access a set of relations as if it were a single table, and limiting their access to just that. Many DBMS don't allow updates to a view, instead you have to perform the updates on the individual tables.

Views can also be used to restrict access to rows (a subset of a particular table). One does not need views to restrict access to columns, as MySQL Server has a sophisticated privilege system. See section General Security Issues and the MySQL Access Privilege System.

In designing our implementation of views, we aim toward (as fully as possible within the confines of SQL) compliance with "Codd's Rule #6" for relational database systems: all views that are theoretically updatable, should in practice also be updatable. This is a complex issue, and we are taking the time to make sure we get it right.

The implementation itself will be done in stages. Unnamed views (derived tables, a subquery in the FROM clause of a SELECT) are already implemented in version 4.1.

Note: If you are an enterprise level user with an urgent need for views, please contact sales@mysql.com to discuss sponsoring options. Targeted financing of this particular effort by one or more companies would allow us to allocate additional resources to it. One example of a feature sponsored in the past is replication. `--' as the Start of a Comment

Some other SQL databases use `--' to start comments. MySQL Server has `#' as the start comment character. You can also use the C comment style /* this is a comment */ with MySQL Server. See section Comment Syntax.

MySQL Server Version 3.23.3 and above support the `--' comment style, provided the comment is followed by a space (or by a control character such as a newline). This is because this 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!

Think about what happens if the value of payment is negative. Because 1--1 is legal in SQL, the consequences of allowing comments to start with `--' are terrible.

Using our implementation of this method of commenting in MySQL Server Version 3.23.3 and up, 1-- This is a comment is actually safe.

Another safe feature is that the mysql command-line client removes all lines that start with `--'.

The following information is relevant only if you are running a MySQL version earlier than 3.23.3:

If you have an 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

1.7.5 How MySQL deals with constraints

As MySQL allows you to work with both transactional and non-transactional tables (which don't allow rollback), constraint handling is a bit different in MySQL than in other databases.

We have to handle the case when you have updated a lot of rows with a non-transactional table which can't rollback on errors.

The basic philosophy is to try to give an error for anything that we can detect on compile time but try to recover from any errors we get run time. We do this in most cases, but not yet for all. See section New Features Planned For The Near Future.

The basic options MySQL has is to stop the statement in the middle or do it's best to recover from the problem and continue.

Here follows what happens with the different types of constraints. Constraint PRIMARY KEY / UNIQUE

Normally you will get an error when you try to INSERT / UPDATE a row that causes a primary key, unique key or foreign key violation. If you are using a transactional storage engine, like InnoDB, MySQL will automatically roll back the transaction. If you are using a non-transactional storage engine MySQL will stop at the wrong row and leave the rest of the rows unprocessed.

To make life easier MySQL has added support for the IGNORE directive to most commands that can cause a key violation (like INSERT IGNORE ...). In this case MySQL will ignore any key violation and continue with processing the next row. You can get information of what MySQL did with the mysql_info() API function and in later MySQL 4.1 version with the SHOW WARNINGS command. See section mysql_info(). See section SHOW WARNINGS | ERRORS.

Note that for the moment only InnoDB tables support foreign keys. See section FOREIGN KEY Constraints. Foreign key support in MyISAM tables is scheduled for inclusion in the MySQL 5.0 source tree. Constraint NOT NULL and DEFAULT values

To be able to support easy handling of non-transactional tables all fields in MySQL have default values.

If you insert a 'wrong' value in a column like a NULL in a NOT NULL column or a too big numerical value in a numerical column, MySQL will instead of giving an error instead set the column to the 'best possible value'. For numerical values this is 0, the smallest possible values or the largest possible value. For strings this is either the empty string or the longest possible string that can be in the column.

This means that if you try to store NULL into a column that doesn't take NULL values, MySQL Server will store 0 or '' (empty string) in it instead. This last behaviour can, for single row inserts, be changed with the -DDONT_USE_DEFAULT_FIELDS compile option.) See section Typical configure Options. This causes INSERT statements to generate an error unless you explicitly specify values for all columns that require a non-NULL value.

The reason for the above rules is that we can't check these conditions before the query starts to execute. If we encounter a problem after updating a few rows, we can't just rollback as the table type may not support this. The option to stop is not that good as in this case the update would be 'half done' which is probably the worst possible scenario. In this case it's better to 'do the best you can' and then continue as if nothing happened. In MySQL 5.0 we plan to improve this by providing warnings for automatic field conversions, plus an option to let you roll back statements that only use transactional tables in case one such statement does a field assignment that is not allowed.

The above means that one should generally not use MySQL to check field content, but instead handle this in the application. Constraint ENUM and SET

In MySQL 4.x ENUM is not a real constrain but a more efficient way to store fields that can only contain a given set of values. This is because of the same reasons NOT NULL is not honoured. See section Constraint NOT NULL and DEFAULT values.

If you insert an wrong value in an ENUM field, it will be set to the reserved enum number 0, which will be displayed as an empty string in string context. See section The ENUM Type.

If you insert an wrong option in a SET field, the wrong value will be ignored. See section The SET Type.

1.7.6 Known Errors and Design Deficiencies in MySQL Errors in 3.23 Fixed in a Later MySQL Version

The following known errors/bugs are not fixed in MySQL 3.23 because fixing them would involves changing a lot of code which could introduce other even worse bugs. The bugs are also classified as 'not fatal' or 'bearable'. Open Bugs / Design Deficiencies in MySQL

The following problems are known and fixing them is a high priority:

The following problems are known and will be fixed in due time:

The following are known bugs in earlier versions of MySQL:

For platform-specific bugs, see the sections about compiling and porting.

1.8 MySQL and The Future (The TODO)

This section summarises the features that we plan to implement in MySQL Server. The lists are broken up per version, and the items are approximately in the order they will be done.

Note: If you are an enterprise level user with an urgent need for a particular feature, please contact sales@mysql.com to discuss sponsoring options. Targeted financing by one or more companies allows us to allocate additional resources for that specific purpose. One example of a feature sponsored in the past is replication.

1.8.1 New Features Planned For 4.1

The features below are not yet implemented in MySQL 4.1, but are planned for implementation before MySQL 4.1 moves into its beta phase. For a list what is already done in MySQL 4.1, see Features Available in MySQL 4.1.

Development of other things has already shifted to the 5.0 tree.

1.8.2 New Features Planned For 5.0

The following features are planned for inclusion into MySQL 5.0. Note that because we have many developers that are working on different projects, there will also be many additional features. There is also a small chance that some of these features will be added to MySQL 4.1. For a list what is already done in MySQL 4.1, see Features Available in MySQL 4.1.

For those wishing to take a look at the bleeding edge of MySQL development, we have made our BitKeeper repository for MySQL version 5.0 publicly available. See section Installing from the Development Source Tree.

Stored Procedures
New functionality
Standards compliance, portability and migration
Speed enhancements
Usability enhancements

1.8.3 New Features Planned For 5.1

New functionality
Speed enhancements
Usability enhancements

1.8.4 New Features Planned For The Near Future

New functionality
Standards compliance, portability and migration
Speed enhancements
Usability enhancements
New operating systems

1.8.5 New Features Planned For The Mid-Term Future

Time is given according to amount of work, not real time.

1.8.6 New Features We Don't Plan To Do

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