MySql Tutorial 5 - Getting Information About Databases and Tables

What if you forget the name of a database or table, or what the
structure of a given table is (for example, what its columns are
called)? MySQL addresses this problem through several statements
that provide information about the databases and tables it
supports.

You have previously seen SHOW DATABASES, which
lists the databases managed by the server. To find out which
database is currently selected, use the
DATABASE() function:

mysql> SELECT DATABASE();
+------------+
| DATABASE() |
+------------+
| menagerie  |
+------------+

If you have not yet selected any database, the result is
NULL.

To find out what tables the default database contains (for
example, when you are not sure about the name of a table), use
this command:

mysql> SHOW TABLES;

+---------------------+
| Tables_in_menagerie |
+---------------------+
| event               |
| pet                 |
+---------------------+

The name of the column in the output produced by this statement is
always
Tables_in_db_name,
where db_name is the name of the
database. See Section 12.5.4.25, “SHOW TABLES Syntax”, for more information.

If you want to find out about the structure of a table, the
DESCRIBE command is useful; it displays
information about each of a table's columns:

mysql> DESCRIBE pet;
+---------+-------------+------+-----+---------+-------+
| Field   | Type        | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| name    | varchar(20) | YES  |     | NULL    |       |
| owner   | varchar(20) | YES  |     | NULL    |       |
| species | varchar(20) | YES  |     | NULL    |       |
| sex     | char(1)     | YES  |     | NULL    |       |
| birth   | date        | YES  |     | NULL    |       |
| death   | date        | YES  |     | NULL    |       |
+---------+-------------+------+-----+---------+-------+

Field indicates the column name,
Type is the data type for the column,
NULL indicates whether the column can contain
NULL values, Key indicates
whether the column is indexed, and Default

specifies the column's default value. Extra
displays special information about columns; for example, if a
column was created with the AUTO_INCREMENT
option, this is shown here.