Getting Information About Databases and Tables
MySQL provides a SHOW statement that has several variant forms that display information about databases and the tables in them. SHOW is helpful for keeping track of the contents of your databases and for reminding yourself about the structure of your tables:
Several forms of the SHOW statement take a LIKE 'pattern' clause allowing a pattern to be given that limits the scope of the output. MySQL interprets 'pattern' as an SQL pattern that may include the '%' and '_' wildcard characters. For example, this statement displays the names of tables in the current database that begin with 'geo':
SHOW TABLES LIKE 'geo%';
To match a literal instance of a wildcard character in a LIKE pattern, precede it with a backslash. Generally, this is done to match a literal '_', which occurs frequently in database, table, and column names.
The mysqldump client program allows you to see the structure of your tables in the form of a CREATE TABLE statement (much like SHOW CREATE TABLE). When using mysqldump to review table structure, be sure to invoke it with the --no-data option so that you don't get swamped with your table's data!
% mysqldump --no-data db_name [tbl_name] ...
If you list only the database name without any table names, mysqldump displays the structure for all tables in the database. Otherwise it shows information for the named tables.
It's sometimes useful to be able to tell from within an application whether a given table exists. You can use SHOW TABLES to find out (but remember that SHOW TABLES does not list TEMPORARY tables):
SHOW TABLES LIKE 'tbl_name'; SHOW TABLES FROM db_name LIKE 'tbl_name';
SELECT COUNT(*) FROM tbl_name; SELECT * FROM tbl_name WHERE 0;
Each statement succeeds if the table exists, and fails if it doesn't. The first statement is most appropriate for MyISAM tables, for which COUNT(*) with no WHERE clause is highly optimized. It's not so good for InnoDB or BDB tables, which require a full scan to count the rows. The second statement is more general because it runs quickly for any storage engine. These statements are most suitable for use within application programming languages such as Perl or PHP because you can test the success or failure of the query and take action accordingly. They're not especially useful in a batch script that you run from mysql because you can't do anything if an error occurs except terminate (or ignore the error, but then there's obviously no point in running the query at all).
Beginning with version 5.0.2, MySQL implements INFORMATION_SCHEMA, which provides another way to obtain information about databases (that is, database metadata). INFORMATION_SCHEMA is based on the SQL standard. That is, the access mechanism is standard, even though some of the content is MySQL-specific. This makes INFORMATION_ SCHEMA more portable than the various SHOW statements, which are entirely MySQL-specific. INFORMATION_SCHEMA is accessed through SELECT statements and can be used in a flexible manner. SHOW statements always display a fixed set of columns and you cannot capture the output in a table. With INFORMATION_SCHEMA, the SELECT statement can name specific output columns and a WHERE clause to specify exactly what information you require. Also, you can use joins or subqueries, and you can save the result of the retrieval in another table for further processing.
You can think of INFORMATION_SCHEMA as a virtual database where the various tables within it are views for different kinds of database metadata. To see what tables INFORMATION_SCHEMA contains, use SHOW TABLES:
mysql> SHOW TABLES IN INFORMATION_SCHEMA; +---------------------------------------+ | Tables_in_information_schema | +---------------------------------------+ | SCHEMATA | | TABLES | | COLUMNS | | CHARACTER_SETS | | COLLATIONS | | COLLATION_CHARACTER_SET_APPLICABILITY | | ROUTINES | | STATISTICS | | VIEWS | | USER_PRIVILEGES | | SCHEMA_PRIVILEGES | | TABLE_PRIVILEGES | | COLUMN_PRIVILEGES | | TABLE_CONSTRAINTS | | KEY_COLUMN_USAGE | +---------------------------------------+
mysql> DESCRIBE INFORMATION_SCHEMA.CHARACTER_SETS; +----------------------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +----------------------+-------------+------+-----+---------+-------+ | CHARACTER_SET_NAME | varchar(64) | NO | | | | | DEFAULT_COLLATE_NAME | varchar(64) | NO | | | | | DESCRIPTION | varchar(60) | NO | | | | | MAXLEN | bigint(3) | NO | | 0 | | +----------------------+-------------+------+-----+---------+-------+
To display information from a table, use a SELECT statement. The general query to see all the columns in any given INFORMATION_SCHEMA table is as follows:
SELECT * FROM INFORMATION_SCHEMA.tbl_name;
mysql> SELECT * FROM INFORMATION_SCHEMA.TABLES -> WHERE TABLE_SCHEMA='sampdb' AND TABLE_NAME='member'\G *************************** 1. row *************************** TABLE_CATALOG: NULL TABLE_SCHEMA: sampdb TABLE_NAME: member TABLE_TYPE: BASE TABLE ENGINE: MyISAM VERSION: 10 ROW_FORMAT: Dynamic TABLE_ROWS: 102 AVG_ROW_LENGTH: 137 DATA_LENGTH: 14000 MAX_DATA_LENGTH: 4294967295 INDEX_LENGTH: 2048 DATA_FREE: 0 AUTO_INCREMENT: 103 CREATE_TIME: 2005-01-20 22:38:03 UPDATE_TIME: 2005-01-20 22:38:23 CHECK_TIME: NULL TABLE_COLLATION: latin1_swedish_ci CHECKSUM: NULL CREATE_OPTIONS: TABLE_COMMENT:
Other INFORMATION_SCHEMA tables might be implemented over time. See the MySQL Reference Manual for the current list of tables.
Along with the implementation of INFORMATION_SCHEMA, several SHOW statements have been extended in MySQL 5.0 to allow a WHERE clause. Even though each SHOW statement still displays a fixed set of columns, WHERE provides more flexibility about specifying which rows to return. The WHERE clause should refer to the columns displayed by the SHOW statement. The WHERE capability has been added to the following SHOW statements:
SHOW DATABASES SHOW TABLES SHOW OPEN TABLES SHOW COLUMNS SHOW KEYS SHOW VARIABLES SHOW STATUS SHOW TABLE STATUS SHOW PROCEDURE STATUS SHOW FUNCTION STATUS SHOW CHARACTER SET SHOW COLLATION