MySQL基础之 如何删除主键
我们在一个表中设置了主键之后，那么如何删除主键呢？
删除主键的语法是：
ALTER TABLE TABLE_NAME DROP PRIMARY KEY;
在这里我们要考虑两种情况：
1、可以直接使用drop删除主键的情况。
mysql> create table test1_3( -> id int not null primary key, -> name char(10) -> ); Query OK, 0 rows affected (0.01 sec) mysql> alter table test1_3 drop primary key; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0
2、如果带有主键的列还有AUTO_INCREMENT属性，需要间接方式去掉。
mysql> create table test1_2( -> id int not null auto_increment, -> name char(10),-> primary key(id) -> ); Query OK, 0 rows affected (0.00 sec) mysql> desc test1_2；+-------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | char(10) | YES | | NULL | | +-------+----------+------+-----+---------+----------------+ 2 rows in set (0.00 sec) mysql> desc test1_2; +-------+----------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------+----------+------+-----+---------+----------------+ | id | int(11) | NO | PRI | NULL | auto_increment | | name | char(10) | YES | | NULL | | +-------+----------+------+-----+---------+----------------+ 2 rows in set (0.00 sec) mysql> alter table test1_2 drop primary key; ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key #这说明此列是自动增长列，无法直接删除 mysql> alter table test1_2 modify id int; Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> alter table test1_2 drop primary key; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0
所以说如果列的属性还带有AUTO_INCREMENT，那么要先将这个列的自动增长属性去掉，才可以删除主键。