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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.
In effect, we have a database named
INFORMATION_SCHEMA, although the server does not create a database directory with that name. It is possible to select
INFORMATION_SCHEMA as the default database with a
USE statement, but it is possible only to read the contents of tables. You cannot insert into them, update them, or delete from them.
Here is an example of a statement that retrieves information from
SELECT table_name, table_type, engine->
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 alphabetical order, showing just three pieces of information: the name of the table, its type, and its storage engine.
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
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 TABLES, and so forth). Using
SELECT has these advantages, compared to
It conforms to Codd's rules. That is, all access is done on tables.
Nobody needs to learn a new statement syntax. Because they already know how
SELECT works, they only need to learn the object names.
The implementor need not worry about adding keywords.
There are millions of possible output variations, instead of just one. This provides more flexibility for applications that have varying requirements about what metadata they need.
Migration is easier because every other DBMS does it this way.
SHOW is popular with MySQL employees and users, 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 20.19, “Extensions to
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
Although other DBMSs use a variety of names, like
system, the standard name is
The following sections describe each of the tables and columns that are 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.”
SHOW Name” indicates the equivalent field name in the closest
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.
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
TABLE_COLLATION in the
TABLES table.) See the list of reserved words near the end of this article: http://www.dbazine.com/gulutzan5.shtml.
The definition for character columns (for example,
TABLES.TABLE_NAME) is generally
N) CHARACTER SET utf8
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. If the default collation is not correct for your needs, you can force a suitable collation with a
COLLATE clause (Section 10.5.1, “Using
COLLATE in SQL Statements”).
Each section indicates what
SHOW statement is equivalent to a
SELECT that retrieves information from
INFORMATION_SCHEMA, if there is such a statement.
Note: At present, there are some missing columns and some columns out of order. We are working on this and updating the documentation as changes are made.
For answers to questions that are often asked concerning the
INFORMATION_SCHEMA database, see Section A.7, “MySQL 5.0 FAQ —
A schema is a database, so the
SCHEMATA table provides information about databases.
DEFAULT_COLLATION_NAME was added in MySQL 5.0.6.
The following statements are equivalent:
SELECT SCHEMA_NAME AS `Database` FROM INFORMATION_SCHEMA.SCHEMATA [WHERE SCHEMA_NAME LIKE '
wild'] SHOW DATABASES [LIKE '
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