Mysql,添加复合唯一索引

CREATE TABLE `t_user` (
  `id` varchar(11) NOT NULL,
  `username` varchar(40) NOT NULL,
  `password` varchar(40) NOT NULL,
  `sex` varchar(4) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

alter  table  t_user add unique index save_unique_index(username,password); -- 添加多字段唯一索引

CREATE TABLE `t_user` ( 
  `id` varchar(11) NOT NULL,
  `username` varchar(40) NOT NULL,
  `password` varchar(40) NOT NULL,
  `sex` varchar(4) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `save_unique_index` (`username`,`password`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

1	Tom	123	男
2	Tom	1234	男
3	Jerry	234	女

insert into t_user(id,username,password,sex) values('4','Tom','123','男'); --  Duplicate entry 'Tom-123' for key 'save_unique_index'

insert into t_user(id,username,password,sex) values('4','Tom','12345','男'); -- 受影响的行: 1

1	Tom	123	男
2	Tom	1234	男
3	Jerry	234	女
4	Tom	12345	男