DROP TABLE IF EXISTS `student`;
CREATE TABLE `student` (
`stu_id` int(11) NOT NULL AUTO_INCREMENT,
`name` varchar(255) DEFAULT NULL,
PRIMARY KEY (`stu_id`),
UNIQUE KEY `UK_student_name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=12 DEFAULT CHARSET=utf8;

create unique index UK_student_name on student (name);

alter table student add constraint uk_student_name unique (name);

mysql> ALTER TABLE student DROP INDEX name; 

Query OK, 0 rows affected (0.85 sec)

alter table user add unique index(user_id,user_name);