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 t1\g *************************** 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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