我们如何将PRIMARY KEY约束应用于现有MySQL表的字段?
我们可以借助ALTERTABLE语句将PRIMARYKEY约束应用于现有MySQL表的列。
语法
ALTER TABLE table_name MODIFY colum_name datatype PRIMARY KEY; OR ALTER TABLE table_name ADD PRIMARY KEY (colum_name);
假设我们有一个名为“Player”的下表,并且我们想将PRIMARYKEY约束添加到“ID”列,那么可以通过ALTERTABLE命令来完成它,如下所示:
mysql> DESCRIBE Player; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | ID | int(11) | YES | | NULL | | | Name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.04 sec) mysql> ALTER TABLE Player MODIFY ID INT PRIMARY KEY; Records: 0 Duplicates: 0 Warnings: 0 mysql> DESCRIBE Player; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | ID | int(11) | NO | PRI | NULL | | | Name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.04 sec)
从以上结果集中可以看出,MySQL在字段“ID”中添加了PRIMARYKEY约束。我们也可以使用以下查询添加PRIMARYKEY约束-
Alter table Player ADD PRIMARY KEY(ID);