mysql表的字段怎么设置唯一性

1、登录mysql，在test数据库中新建一个表，主键是id，后面把chengshi字段设置为唯一性：CREATETABLE`chengshi`(`id`int(11)NOTNULLAUTO_INCREMENT,`shengfen`varchar(10)DEFAULTNULL,`chengshi`varchar(10)DEFAULTNULL,`shijian`timestampNOTNULLDEFAULTCURRENT_TIMESTAMPONUPDATECURRENT_TIMESTAMP,PRIMARYKEY(`id`))ENGINE=InnoDBAUTO_INCREMENT=10DEFAULTCHARSET=utf8

2、在表中增加一些数据：insert into `chengshi` (`id`, `shengfen`, `chengshi`, `shijian`) values('1','河南','安阳','2019-03-05 10:14:27');insert into `chengshi` (`id`, `shengfen`, `chengshi`, `shijian`) values('2','河南','郑州','2019-03-05 10:14:27');insert into `chengshi` (`id`, `shengfen`, `chengshi`, `shijian`) values('3','河南','洛阳','2019-03-05 10:14:27');insert into `chengshi` (`id`, `shengfen`, `chengshi`, `shijian`) values('4','河南','漯河','2019-03-05 10:14:27');insert into `chengshi` (`id`, `shengfen`, `chengshi`, `shijian`) values('5','河南','新乡','2019-03-05 10:14:27');insert into `chengshi` (`id`, `shengfen`, `chengshi`, `shijian`) values('6','河南','濮阳','2019-03-05 10:14:27');insert into `chengshi` (`id`, `shengfen`, `chengshi`, `shijian`) values('7','河南','焦作','2019-03-05 10:14:27');insert into `chengshi` (`id`, `shengfen`, `chengshi`, `shijian`) values('8','河南','鹤壁','2019-03-05 10:14:27');insert into `chengshi` (`id`, `shengfen`, `chengshi`, `shijian`) values('9','河南','开封','2019-03-05 10:14:27');

3、把chengshi字段设置为唯一性：ALTER TABLE `test`.`chengshi` ADD UNIQUE INDEX `chengshi` (`chengshi`);

4、查询chengshi表的数据：SELECT * FROM chengshi;

5、把id=1的城市修改为郑州，出错有重复：UPDATE chengshi SET chengshi='郑州' WHERE id=1;

6、增加一个城市安阳，出现重复：INSERT INTO chengshi(shengfen,chengshi)VALUES('河南','安阳');