如何在现有 MySQL 表的多个列上设置 PRIMARY KEY?
mysqlmysqli database
我们可以通过使用 ADD 关键字和 ALTER TABLE 语句在现有表的多个列上设置 PRIMARY KEY 约束。
示例
假设我们有一个表"Room_allotment"，如下所示 −
mysql> Create table Room_allotment(Id Int, Name Varchar(20), RoomNo Int); Query OK, 0 rows affected (0.20 sec) mysql> Describe Room_allotment; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | Id | int(11) | YES | | NULL | | | Name | varchar(20) | YES | | NULL | | | RoomNo | int(11) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 3 rows in set (0.11 sec)
现在我们可以使用以下查询在多个列‘id’和‘Name’上添加复合 PRIMARY KEY
mysql> Alter Table Room_allotment ADD PRIMARY KEY(Id, Name); Query OK, 0 rows affected (0.29 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> Describe Room_allotment; +--------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+-------------+------+-----+---------+-------+ | Id | int(11) | NO | PRI | 0 | | | Name | varchar(20) | NO | PRI | | | | RoomNo | int(11) | YES | | NULL | | +--------+-------------+------+-----+---------+-------+ 3 rows in set (0.12 sec)
从上面的结果集中可以观察到，在多个列上添加了PRIMARY KEY。
