如何在 MySQL 中形成一个唯一的复合键?
mysqlmysqli database
要形成唯一的复合键，您需要使用 ADD UNIQUE 命令。以下是语法 −
alter table yourTableName add unique yourUniqueName( yourColumnName1,yourColumnName2,.......N);
让我们首先创建一个表。以下是查询 −
mysql> create table makeCompositeKeyDemo -> ( -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, -> StudentName varchar(40), -> StudentAge int, -> StudentGrade char(1) -> ); Query OK, 0 rows affected (2.34 sec)
现在使用 DESC 命令检查表的描述。以下是查询 −
mysql> desc makeCompositeKeyDemo;
这将产生以下输出 −
+--------------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+----------------+ | Id | int(11) | NO | PRI | NULL | auto_increment | | StudentName | varchar(40) | YES | | NULL | | | StudentAge | int(11) | YES | | NULL | | | StudentGrade | char(1) | YES | | NULL | | +--------------+-------------+------+-----+---------+----------------+ 4 rows in set (1.65 sec)
以下是形成唯一复合键的查询 −
mysql> alter table makeCompositeKeyDemo add unique Name_Age_Grade( StudentName,StudentAge,StudentGrade); Query OK, 0 rows affected (0.69 sec) Records: 0 Duplicates: 0 Warnings: 0
现在再次检查表描述。以下是查询 −
mysql> desc makeCompositeKeyDemo;
这将产生以下输出 −
+--------------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +--------------+-------------+------+-----+---------+----------------+ | Id | int(11) | NO | PRI | NULL | auto_increment | | StudentName | varchar(40) | YES | MUL | NULL | | | StudentAge | int(11) | YES | | NULL | | | StudentGrade | char(1) | YES | | NULL | | +--------------+-------------+------+-----+---------+----------------+ 4 rows in set (0.00 sec)
