mysql无法删除表中的主键

1、mysql删除表中的主键时报错，如下代码所示：

mysql> alter table student drop primary key;ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key

2、问题原因：

查看student表的类型时，发现主键列中有auto_increment(递增)类型选项。如要删除表中的主键，需要先删除auto_increment类型。代码如下所示：

mysql> desc student;+-------+-------------+------+-----+---------+----------------+| Field | Type        | Null | Key | Default | Extra          |+-------+-------------+------+-----+---------+----------------+| id    | int(11)     | NO   | PRI | NULL    | auto_increment || name  | char(20)    | NO   | MUL | NULL    |                || age   | tinyint(2)  | NO   | MUL | 0       |                || dept  | varchar(16) | YES  |     | NULL    |                |+-------+-------------+------+-----+---------+----------------+4 rows in set (0.02 sec)

3、删除student表中的主键列上面的auto_increment类型。代码如下所示：

mysql> alter table student change id id int;Query OK, 0 rows affected (0.04 sec)Records: 0  Duplicates: 0  Warnings: 0

提示：alter table student change id id int;命令修改student表中列的类型，auto_increment类型自然就会被删除。

4、查看student表的类型，发现auto_increment类型已被删除。代码如下所示：

mysql> desc student;+-------+-------------+------+-----+---------+-------+| Field | Type        | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id    | int(11)     | NO   | PRI | 0       |       || name  | char(20)    | NO   | MUL | NULL    |       || age   | tinyint(2)  | NO   | MUL | 0       |       || dept  | varchar(16) | YES  |     | NULL    |       |+-------+-------------+------+-----+---------+-------+4 rows in set (0.01 sec)

5、删除student中的主键。代码如下所示：

mysql> alter table student drop primary key;Query OK, 0 rows affected (0.10 sec)Records: 0  Duplicates: 0  Warnings: 0

6、最后查看student表的类型，发现表中的主键已被删除。代码如下：

mysql> desc student;+-------+-------------+------+-----+---------+-------+| Field | Type        | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id    | int(11)     | NO   |     | 0       |       || name  | char(20)    | NO   | MUL | NULL    |       || age   | tinyint(2)  | NO   | MUL | 0       |       || dept  | varchar(16) | YES  |     | NULL    |       |+-------+-------------+------+-----+---------+-------+4 rows in set (0.00 sec)