前言

在MySQL 8.0版本中，引入了一个非常有用的新特性 ― 检查性约束，它可以提高对非法或不合理数据写入的控制能力；接下来我们就来详细了解一下。

检查性约束

创建、删除与查看

(1)可以在建表时，创建检查性约束

mysql> CREATE TABLE t1

-> (

-> CHECK (c1 <> c2),

-> c1 INT CHECK (c1 > 10),

-> c2 INT CONSTRAINT c2_positive CHECK (c2 > 0),

-> c3 INT CHECK (c3 < 100),

-> CONSTRAINT c1_nonzero CHECK (c1 <> 0),

-> CHECK (c1 > c3)

-> );

Query OK, 0 rows affected (0.03 sec)

(2)也可以通过下列语句，新增检查性约束

mysql> ALTER TABLE t1 ADD CONSTRAINT c3_nonzero CHECK ((c3<>0));

Query OK, 0 rows affected (0.16 sec)

Records: 0 Duplicates: 0 Warnings: 0

(3)可以通过下列语句，删除检查性约束

mysql> ALTER TABLE t1 DROP CONSTRAINT c3_nonzero;

Query OK, 0 rows affected (0.02 sec)

Records: 0 Duplicates: 0 Warnings: 0

(4)可以通过查询表结构的方式，查看检查性约束

mysql> SHOW CREATE TABLE t1G

*************************** 1. row ***************************

Table: t1

Create Table: CREATE TABLE `t1` (

`c1` int DEFAULT NULL,

`c2` int DEFAULT NULL,

`c3` int DEFAULT NULL,

CONSTRAINT `c1_nonzero` CHECK ((`c1` <> 0)),

CONSTRAINT `c2_positive` CHECK ((`c2` > 0)),

CONSTRAINT `t1_chk_1` CHECK ((`c1` <> `c2`)),

CONSTRAINT `t1_chk_2` CHECK ((`c1` > 10)),

CONSTRAINT `t1_chk_3` CHECK ((`c3` < 100)),

CONSTRAINT `t1_chk_4` CHECK ((`c1` > `c3`))

) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

1 row in set (0.00 sec)

(5)也可以通过下面两个视图查看，其中table_constraints查询表存在哪些约束，check_constraints查询检查性约束的具体定义

mysql> SELECT * FROM information_schema.table_constraints WHERE table_name="t1";

+--------------------+-------------------+-----------------+--------------+------------+-----------------+----------+

| CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME | TABLE_SCHEMA | TABLE_NAME | CONSTRAINT_TYPE | ENFORCED |

+--------------------+-------------------+-----------------+--------------+------------+-----------------+----------+

| def | test | c1_nonzero | test | t1 | CHECK | YES |

| def | test | c2_positive | test | t1 | CHECK | YES |

| def | test | t1_chk_1 | test | t1 | CHECK | YES |

| def | test | t1_chk_2 | test | t1 | CHECK | YES |

| def | test | t1_chk_3 | test | t1 | CHECK | YES |

| def | test | t1_chk_4 | test | t1 | CHECK | YES |

+--------------------+-------------------+-----------------+--------------+------------+-----------------+----------+

6 rows in set (0.00 sec)

mysql> SELECT * FROM information_schema.check_constraints WHERE constraint_name="c1_nonzero";

+--------------------+-------------------+-----------------+--------------+

| CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME | CHECK_CLAUSE |

+--------------------+-------------------+-----------------+--------------+

| def | test | c1_nonzero | (`c1` <> 0) |

+--------------------+-------------------+-----------------+--------------+

1 row in set (0.00 sec)

(6)当插入不符合检查性约束的数据时，会直接报错

mysql> insert into t1 values(0,0,0);

ERROR 3819 (HY000): Check constraint "c1_nonzero" is violated.

限制

(1)自增列和其他表的列，不支持检查性约束

(2)不确定的函数，如CONNECTION_ID(),CURRENT_USER(),NOW()等，不支持检查性约束

(3)用户自定义函数，不支持检查性约束

(4)存储过程，不支持检查性约束

(5)变量，不支持检查性约束

(6)子查询，不支持检查性约束

总结

检查性约束，还是一个非常不错的功能，可以实现丰富的数据校验场景，大家可以尝试一下。

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