mysql 唯一键
唯一键特点:
1、唯一键在一张表中可以有多个。
2、唯一键允许字段数据为NULL,NULL可以有多个(NULL不参与比较)
//一个表中允许存在多个唯一键,唯一键允许为空,在不为空的情况下,不允许重复
//设置一个字段为唯一键
mysql> alter table `table1` add unique ( `name_new`); Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql> desc table1; +------------+------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +------------+------------------+------+-----+---------+----------------+ | name_new | char(32) | YES | UNI | NULL | | | transactor | varchar(10) | NO | | NULL | | | pid | int(10) unsigned | NO | PRI | NULL | auto_increment | +------------+------------------+------+-----+---------+----------------+ 3 rows in set (0.00 sec) //删除唯一键(该字段依然存在) alter table table1 drop index name_new;