mysql中unique和primary key的区别

mysql> create table user (name varchar(255),constraint name_un unique(name));

Query OK, 0 rows affected (0.07 sec)

mysql> insert user values("张三");

Query OK, 1 row affected (0.02 sec)

mysql> insert user values("张三");

ERROR 1062 (23000): Duplicate entry '张三' for key 'user.name_un'

mysql> alter table user drop index name_un;

Query OK, 0 rows affected (0.03 sec)

Records: 0  Duplicates: 0  Warnings: 0

mysql> insert user values("张三");

Query OK, 1 row affected (0.02 sec)