INFORMATION_SCHEMA is a database within each MySQL instance, the place that stores information about all the other databases that the MySQL server maintains. INFORMATION_SCHEMA是每个MySQL实例中的一个数据库，存储MySQL服务器维护的所有其他数据库的信息。The INFORMATION_SCHEMA database contains several read-only tables. INFORMATION_SCHEMA数据库包含几个只读表。They are actually views, not base tables, so there are no files associated with them, and you cannot set triggers on them. 它们实际上是视图，而不是基表，因此没有与它们关联的文件，并且不能对它们设置触发器。Also, there is no database directory with that name.此外，没有使用该名称的数据库目录。

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.虽然可以使用USE语句选择INFORMATION_SCHEMA作为默认数据库，但只能读取表的内容，不能对表执行INSERT、UPDATE或DELETE操作。

Here is an example of a statement that retrieves information from INFORMATION_SCHEMA:下面是一个从INFORMATION_SCHEMA中检索信息的语句示例：

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.说明：该语句请求数据库db5中所有表的列表，只显示三条信息：表的名称、类型和存储引擎。

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. 由于某些MySQL对象表示为文件，因此在INFORMATION_SCHEMA字符串列中的搜索可能会受到文件系统大小写敏感度的影响。For more information, see Section 10.8.7, “Using Collation in INFORMATION_SCHEMA Searches”.有关更多信息，请参阅第10.8.7节，“在INFORMATION_SCHEMA搜索中使用排序规则”。

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:

Because SHOW is familiar and widely used, the SHOW statements remain as an alternative. 由于SHOW是一个熟悉且广泛使用的节目，SHOW语句仍然是一个替代方案。In fact, along with the implementation of INFORMATION_SCHEMA, there are enhancements to SHOW as described in Section 26.8, “Extensions to SHOW Statements”.

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. 对于大多数INFORMATION_SCHEMA，每个MySQL用户都有权访问这些表，但只能看到表中与用户具有适当访问权限的对象对应的行。In some cases (for example, the ROUTINE_DEFINITION column in the INFORMATION_SCHEMA ROUTINES 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, InnoDB 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 SHOW statements. In either case, you must have some privilege on an object to see information about it.在这两种情况下，您必须对某个对象拥有一定的权限才能查看有关该对象的信息。

INFORMATION_SCHEMA queries that search for information from more than one database might take a long time and impact performance. 从多个数据库中搜索信息的INFORMATION_SCHEMA查询可能需要很长时间，并会影响性能。To check the efficiency of a query, you can use EXPLAIN. 要检查查询的效率，可以使用EXPLAIN。For information about using EXPLAIN output to tune INFORMATION_SCHEMA queries, see Section 8.2.3, “Optimizing INFORMATION_SCHEMA Queries”.有关使用EXPLAIN输出优化INFORMATION_SCHEMA查询的信息，请参阅第8.2.3节，“优化INFORMATION_SCHEMA查询”。

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. SQL Server 2000（也遵循该标准）的用户可能会注意到一种强烈的相似性。However, MySQL has omitted many columns that are not relevant for our implementation, and added columns that are MySQL-specific. 然而，MySQL省略了许多与实现无关的列，并添加了特定于MySQL的列。One such added 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.

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: https://web.archive.org/web/20070428032454/http://www.dbazine.com/db2/db2-disarticles/gulutzan5.

The following sections describe each of the tables and columns in INFORMATION_SCHEMA. 以下各节介绍INFORMATION_SCHEMA中的每个表和列。For each column, there are three pieces of information:对于每一列，有三条信息：

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 = SCHEMA() condition to the WHERE clause of a query that retrieves information from an INFORMATION_SCHEMA table.

These sections discuss additional INFORMATION_SCHEMA-related topics:这些部分讨论与INFORMATION_SCHEMA相关的其他内容：