首先看下面这段MySQL的操作,我新建了一个含有a和b的表,其中a用check约束必须大于0,然而我插入了一条(-2,1,1)的数据,其中a=-2,也是成功插入的。
所以MySQL只是check,但是不强制check。
1 mysql> create table checkDemoTable(a int,b int,id int,primary key(id)); 2 Query OK, 0 rows affected 3 4 mysql> alter table checkDemoTable add constraint checkDemoConstraint check(a>0); 5 Query OK, 0 rows affected 6 Records: 0 Duplicates: 0 Warnings: 0 7 8 mysql> insert into checkDemoTable values(-2,1,1); 9 Query OK, 1 row affected 10 11 mysql> select * from checkDemoTable; 12 +----+---+----+ 13 | a | b | id | 14 +----+---+----+ 15 | -2 | 1 | 1 | 16 +----+---+----+ 17 1 row in set
解决这个问题有两种办法:
1. 如果需要设置CHECK约束的字段范围小,并且比较容易列举全部的值,就可以考虑将该字段的类型设置为枚举类型 enum()或集合类型set()。比如性别字段可以这样设置,插入枚举值以外值的操作将不被允许。
1 mysql> create table checkDemoTable(a enum('男','女'),b int,id int,primary key(id)); 2 Query OK, 0 rows affected 3 4 mysql> insert into checkDemoTable values('男',1,1); 5 Query OK, 1 row affected 6 7 mysql> select * from checkDemoTable; 8 +----+---+----+ 9 | a | b | id | 10 +----+---+----+ 11 | 男 | 1 | 1 | 12 +----+---+----+ 13 1 row in set
2. 如果需要设置CHECK约束的字段范围大,且列举全部值比较困难,比如:>0的值,那就只能使用触发器来代替约束实现数据的有效性了。如下代码,可以保证a>0。
1 mysql> create table checkDemoTable(a int,b int,id int,primary key(id)); 2 Query OK, 0 rows affected 3 4 mysql> delimiter || 5 drop trigger if exists checkTrigger|| 6 create trigger checkTrigger before insert on checkDemoTable for each row 7 begin 8 if new.a<=0 then set new.a=1; end if; 9 end|| 10 delimiter; 11 12 Query OK, 0 rows affected 13 Query OK, 0 rows affected 14 15 mysql> insert into checkDemoTable values(-1,1,1); 16 Query OK, 1 row affected 17 18 mysql> select * from checkDemoTable; 19 +---+---+----+ 20 | a | b | id | 21 +---+---+----+ 22 | 1 | 1 | 1 | 23 +---+---+----+ 24 1 row in set