mysql> SHOW COLUMNS FROM number;

+---------+----------------------+------+-----+---------+----------------+

| Field   | Type                 | Null | Key | Default | Extra          |

+---------+----------------------+------+-----+---------+----------------+

| nunbers | smallint(5) unsigned | NO   |     | NULL    |                |

| id      | smallint(5) unsigned | NO   | PRI | NULL    | auto_increment |

+---------+----------------------+------+-----+---------+----------------+

2 rows in set (0.03 sec)

mysql> ALTER TABLE id DROP PRIMARY KEY;

ERROR 1075 (42000): Incorrect table definition; there can be only one auto colum

n and it must be defined as a key