Simple tutorial for Unique Indexes in MySQL _ MySQL

This article mainly introduces a simple tutorial on the unique index in MySQL, which is the basic knowledge in MySQL getting started, for more information, see the UNIQUE mysql index UNIQUE. it is generally used for non-duplicate data fields. we often set the id in the data table as the UNIQUE index UNIQUE, next I will introduce how to use the UNIQUE index UNIQUE in mysql.
The purpose of creating a unique index is not to speed up access, but to avoid data duplication. A unique index can have multiple values but the values of the index column must be unique. The value of the index column can have null values. If you can determine that a data column will only contain different values, you should use the keyword UNIQUE when creating an index for this data column.

Define it as a unique index.

Directly set when creating a table:

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 a unique index:

create unique index UK_student_name on student (name);

Add constraints after creating a table:

alter table student add constraint uk_student_name unique (name);

If you do not need a unique index, you can delete it like this.

mysql> ALTER TABLE student DROP INDEX name;

Query OK, 0 rows affected (0.85 sec)

If you want to add an index

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

The above is the only index in MySQL simple learning tutorial _ MySQL content, for more information, please follow the PHP Chinese network (www.php1.cn )!