我们如何在现有 MySQL 表的多个列上设置 PRIMARY KEY？

我们可以通过使用 ADD 关键字和 ALTER TABLE 语句对现有表的多列设置 PRIMARY KEY 约束。

示例

假设我们有一个表“Room_allotment”如下 -

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”上添加复合主键

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看出，多列都添加了PRIMARY KEY。