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。