Chapter 19 INFORMATION_SCHEMA Tables

Table of Contents

19.1 The INFORMATION_SCHEMA CHARACTER_SETS Table
19.2 The INFORMATION_SCHEMA COLLATIONS Table
19.3 The INFORMATION_SCHEMA COLLATION_CHARACTER_SET_APPLICABILITY Table
19.4 The INFORMATION_SCHEMA COLUMNS Table
19.5 The INFORMATION_SCHEMA COLUMN_PRIVILEGES Table
19.6 The INFORMATION_SCHEMA KEY_COLUMN_USAGE Table
19.7 The INFORMATION_SCHEMA PROFILING Table
19.8 The INFORMATION_SCHEMA ROUTINES Table
19.9 The INFORMATION_SCHEMA SCHEMATA Table
19.10 The INFORMATION_SCHEMA SCHEMA_PRIVILEGES Table
19.11 The INFORMATION_SCHEMA STATISTICS Table
19.12 The INFORMATION_SCHEMA TABLES Table
19.13 The INFORMATION_SCHEMA TABLE_CONSTRAINTS Table
19.14 The INFORMATION_SCHEMA TABLE_PRIVILEGES Table
19.15 The INFORMATION_SCHEMA TRIGGERS Table
19.16 The INFORMATION_SCHEMA USER_PRIVILEGES Table
19.17 The INFORMATION_SCHEMA VIEWS Table
19.18 Extensions to SHOW Statements

INFORMATION_SCHEMA provides access to database metadata.

Metadata is data about the data, such as the name of a database or table, the data type of a column, or access privileges. Other terms that sometimes are used for this information are data dictionary and system catalog.

INFORMATION_SCHEMA is the information database, the place that stores information about all the other databases that the MySQL server maintains. Inside INFORMATION_SCHEMA there are 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 USE statement, you can only read the contents of tables, not perform INSERT, UPDATE, or 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 DESC;
+------------+------------+--------+
| table_name | table_type | engine |
+------------+------------+--------+
| v56        | VIEW       | NULL   |
| v3         | VIEW       | NULL   |
| v2         | VIEW       | NULL   |
| v          | VIEW       | NULL   |
| tables     | BASE TABLE | MyISAM |
| t7         | BASE TABLE | MyISAM |
| t3         | BASE TABLE | MyISAM |
| t2         | BASE TABLE | MyISAM |
| t          | BASE TABLE | MyISAM |
| pk         | BASE TABLE | InnoDB |
| loop       | BASE TABLE | MyISAM |
| kurs       | BASE TABLE | MyISAM |
| k          | BASE TABLE | MyISAM |
| into       | BASE TABLE | MyISAM |
| goto       | BASE TABLE | MyISAM |
| fk2        | BASE TABLE | InnoDB |
| fk         | BASE TABLE | InnoDB |
+------------+------------+--------+
17 rows in set (0.01 sec)

Explanation: The statement requests a list of all the tables in database db5, in reverse alphabetic order, showing just three pieces of information: the name of the table, its type, and its storage engine.

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. Values such as table names in INFORMATION_SCHEMA columns are treated as strings, not identifiers, and are not compared using the identifier rules described in Section 9.2.2, “Identifier Case Sensitivity”. If the result of a string operation on an INFORMATION_SCHEMA column differs from expectations, a workaround is to use an explicit COLLATE clause to force a suitable collation (Section 10.1.7.2, “Using COLLATE in SQL Statements”). You can also use the UPPER() or LOWER() function. For example, in a WHERE clause, you might use:

WHERE TABLE_NAME COLLATE utf8_bin = 'City'
WHERE TABLE_NAME COLLATE utf8_general_ci = 'city'
WHERE UPPER(TABLE_NAME) = 'CITY'
WHERE LOWER(TABLE_NAME) = 'city'

Each MySQL user has the right to access these tables, 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.ROUTINES table), users who have insufficient privileges will see NULL.

The SELECT ... FROM INFORMATION_SCHEMA statement is intended as a more consistent way to provide access to the information provided by the various SHOW statements that MySQL supports (SHOW DATABASES, SHOW TABLES, and so forth). Using SELECT has these advantages, compared to SHOW:

However, because SHOW is popular and because it might be confusing were it to disappear, the advantages of conventional syntax are not a sufficient reason to eliminate SHOW. In fact, along with the implementation of INFORMATION_SCHEMA, there are enhancements to SHOW as well. These are described in Section 19.18, “Extensions to SHOW Statements”.

There is no difference between the privileges required for SHOW statements and those required to select information from INFORMATION_SCHEMA. In either case, you have to have some privilege on an object in order to see information about it.

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 column is the ENGINE column in the INFORMATION_SCHEMA.TABLES table.

Although other DBMSs use a variety of names, like syscat or system, the standard name is INFORMATION_SCHEMA.

The following sections describe each of the tables and columns that are in INFORMATION_SCHEMA. For each column, there are three pieces of information:

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 TABLES table.) See the list of reserved words near the end of this article: http://web.archive.org/web/20070409075643rn_1/www.dbazine.com/db2/db2-disarticles/gulutzan5.

Many sections indicate what SHOW statement is equivalent to a SELECT that retrieves information from INFORMATION_SCHEMA. For 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 = DATABASE() condition to the WHERE clause of a query that retrieves information from an INFORMATION_SCHEMA table.

For answers to questions that are often asked concerning the INFORMATION_SCHEMA database, see Section B.7, “MySQL 5.0 FAQ: INFORMATION_SCHEMA.