'INFORMATION_SCHEMA' provides access to database metadata, information about the MySQL server such as the name of a database or table, the data type of a column, or access privileges. Other terms that are sometimes used for this information are data dictionary and system catalog.
*note information-schema-usage-notes::
*note information-schema-character-set-considerations::
*note information-schema-as-show-alternative::
*note information-schema-privileges::
*note information-schema-performance-considerations::
*note information-schema-standards-considerations::
*note information-schema-conventions::
*note information-schema-related-information::
INFORMATION_SCHEMA Usage Notes
'INFORMATION_SCHEMA' is a database within each MySQL instance, the place that stores information about all the other databases that the MySQL server maintains. The 'INFORMATION_SCHEMA' database contains several read-only tables. They are actually views, not base tables, so there are no files associated with them, and you cannot set triggers on them. Also, there is no database directory with that name.
Although you can select 'INFORMATION_SCHEMA' as the default database with a note 'USE': use. statement, you can only read the contents of tables, not perform note 'INSERT': insert, note 'UPDATE': update, or note 'DELETE': delete. operations on them.
Here is an example of a statement that retrieves information from 'INFORMATION_SCHEMA':
mysql> SELECT table_name, table_type, engine
FROM information_schema.tables
WHERE table_schema = 'db5'
ORDER BY table_name;
+------------+------------+--------+
| table_name | table_type | engine |
+------------+------------+--------+
| fk | BASE TABLE | InnoDB |
| fk2 | BASE TABLE | InnoDB |
| goto | BASE TABLE | MyISAM |
| into | BASE TABLE | MyISAM |
| k | BASE TABLE | MyISAM |
| kurs | BASE TABLE | MyISAM |
| loop | BASE TABLE | MyISAM |
| pk | BASE TABLE | InnoDB |
| t | BASE TABLE | MyISAM |
| t2 | BASE TABLE | MyISAM |
| t3 | BASE TABLE | MyISAM |
| t7 | BASE TABLE | MyISAM |
| tables | BASE TABLE | MyISAM |
| v | VIEW | NULL |
| v2 | VIEW | NULL |
| v3 | VIEW | NULL |
| v56 | VIEW | NULL |
+------------+------------+--------+
17 rows in set (0.01 sec)Explanation: The statement requests a list of all the tables in database 'db5', showing just three pieces of information: the name of the table, its type, and its storage engine.
Character Set Considerations
The definition for character columns (for example, 'TABLES.TABLE_NAME') is generally 'VARCHAR(N) CHARACTER SET utf8' where N is at least 64. MySQL uses the default collation for this character set ('utf8_general_ci') for all searches, sorts, comparisons, and other string operations on such columns.
Because some MySQL objects are represented as files, searches in 'INFORMATION_SCHEMA' string columns can be affected by file system case sensitivity. For more information, see *note charset-collation-information-schema::.
INFORMATION_SCHEMA as Alternative to SHOW Statements
The 'SELECT ... FROM INFORMATION_SCHEMA' statement is intended as a more consistent way to provide access to the information provided by the various note 'SHOW': show. statements that MySQL supports (note 'SHOW DATABASES': show-databases, note 'SHOW TABLES': show-tables, and so forth). Using note 'SELECT': select. has these advantages, compared to *note 'SHOW': show.:
It conforms to Codd's rules, because all access is done on tables.
You can use the familiar syntax of the *note 'SELECT': select. statement, and only need to learn some table and column names.
The implementor need not worry about adding keywords.
You can filter, sort, concatenate, and transform the results from 'INFORMATION_SCHEMA' queries into whatever format your application needs, such as a data structure or a text representation to parse.
This technique is more interoperable with other database systems. For example, Oracle Database users are familiar with querying tables in the Oracle data dictionary.
Because note 'SHOW': show. is familiar and widely used, the note 'SHOW': show. statements remain as an alternative. In fact, along with the implementation of 'INFORMATION_SCHEMA', there are enhancements to note 'SHOW': show. as described in note extended-show::.
INFORMATION_SCHEMA and Privileges
For most 'INFORMATION_SCHEMA' tables, each MySQL user has the right to access them, but can see only the rows in the tables that correspond to objects for which the user has the proper access privileges. In some cases (for example, the 'ROUTINE_DEFINITION' column in the 'INFORMATION_SCHEMA' note 'ROUTINES': information-schema-routines-table. table), users who have insufficient privileges see 'NULL'. Some tables have different privilege requirements; for these, the requirements are mentioned in the applicable table descriptions. For example, note 'InnoDB': innodb-storage-engine. tables (tables with names that begin with 'INNODB_') require the 'PROCESS' privilege.
The same privileges apply to selecting information from 'INFORMATION_SCHEMA' and viewing the same information through *note 'SHOW': show. statements. In either case, you must have some privilege on an object to see information about it.
Performance Considerations
'INFORMATION_SCHEMA' queries that search for information from more than one database might take a long time and impact performance. To check the efficiency of a query, you can use note 'EXPLAIN': explain. For information about using note 'EXPLAIN': explain. output to tune 'INFORMATION_SCHEMA' queries, see *note information-schema-optimization::.
Standards Considerations
The implementation for the 'INFORMATION_SCHEMA' table structures in MySQL follows the ANSI/ISO SQL:2003 standard Part 11 'Schemata'. Our intent is approximate compliance with SQL:2003 core feature F021 'Basic information schema'.
Users of SQL Server 2000 (which also follows the standard) may notice a strong similarity. However, MySQL has omitted many columns that are not relevant for our implementation, and added columns that are MySQL-specific. One such added column is the 'ENGINE' column in the 'INFORMATION_SCHEMA' *note 'TABLES': information-schema-tables-table. table.
Although other DBMSs use a variety of names, like 'syscat' or 'system', the standard name is 'INFORMATION_SCHEMA'.
To avoid using any name that is reserved in the standard or in DB2, SQL Server, or Oracle, we changed the names of some columns marked 'MySQL extension'. (For example, we changed 'COLLATION' to 'TABLE_COLLATION' in the *note 'TABLES': information-schema-tables-table. table.) See the list of reserved words near the end of this article: https://web.archive.org/web/20070428032454/http://www.dbazine.com/db2/db2-disarticles/gulutzan5.
Conventions in the INFORMATION_SCHEMA Reference Sections
The following sections describe each of the tables and columns in 'INFORMATION_SCHEMA'. For each column, there are three pieces of information:
''INFORMATION_SCHEMA' Name' indicates the name for the column in the 'INFORMATION_SCHEMA' table. This corresponds to the standard SQL name unless the 'Remarks' field says 'MySQL extension.'
'note 'SHOW': show. Name' indicates the equivalent field name in the closest note 'SHOW': show. statement, if there is one.
'Remarks' provides additional information where applicable. If this field is 'NULL', it means that the value of the column is always 'NULL'. If this field says 'MySQL extension,' the column is a MySQL extension to standard SQL.
Many sections indicate what note 'SHOW': show. statement is equivalent to a note 'SELECT': select. that retrieves information from 'INFORMATION_SCHEMA'. For *note 'SHOW': show. statements that display information for the default database if you omit a 'FROM DB_NAME' clause, you can often select information for the default database by adding an 'AND TABLE_SCHEMA = SCHEMA()' condition to the 'WHERE' clause of a query that retrieves information from an 'INFORMATION_SCHEMA' table.
Related Information
These sections discuss additional 'INFORMATION_SCHEMA'-related topics:
information about 'INFORMATION_SCHEMA' tables specific to the note 'InnoDB': innodb-storage-engine. storage engine: note innodb-information-schema-tables::
information about 'INFORMATION_SCHEMA' tables specific to the thread pool plugin: *note thread-pool-information-schema-tables::
information about 'INFORMATION_SCHEMA' tables specific to the 'CONNECTION_CONTROL' plugin: *note connection-control-information-schema-tables::
Answers to questions that are often asked concerning the 'INFORMATION_SCHEMA' database: *note faqs-information-schema::
'INFORMATION_SCHEMA' queries and the optimizer: *note information-schema-optimization::
The effect of collation on 'INFORMATION_SCHEMA' comparisons: *note charset-collation-information-schema::
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