在 MySQL 中，直到 MySQL 8.0.16 版本之前，标准 SQL 的 CHECK 约束并未被完全支持。然而，从 MySQL 8.0.16 开始，CHECK 约束在表定义中得到了支持，允许你定义列中必须满足的条件。
使用 CHECK 约束，你可以确保在插入或更新记录时，某列或某组列的值满足特定的条件。
以下是如何在 MySQL 中使用 CHECK 约束
MySQL5.7.X CHECK 约束是无效的，只做检查CHECK ，不强制CHECK
在这个示例中，我们创建了一个名为 t_check 的表，其中有一个c_age 列。CHECK约束确保c_age 列的值在大于等于18。
select @@version 检查MySQL版本信息
mysql> select @@version; +------------+ | @@version | +------------+ | 5.7.28-log | +------------+ 1 row in set (0.00 sec) mysql> status -------------- mysql Ver 14.14 Distrib 5.7.28, for linux-glibc2.12 (x86_64) using EditLine wrapper Connection id: 2 Current database: superdb Current user: root@localhost SSL: Not in use Current pager: stdout Using outfile: '' Using delimiter: ; Server version: 5.7.28-log MySQL Community Server (GPL) Protocol version: 10 Connection: Localhost via UNIX socket Server characterset: utf8 Db characterset: utf8 Client characterset: utf8 Conn. characterset: utf8 UNIX socket: /tmp/mysql.sock Uptime: 6 min 59 sec Threads: 1 Questions: 18 Slow queries: 0 Opens: 111 Flush tables: 1 Open tables: 26 Queries per second avg: 0.042 --------------
create table t_check ( id int not null primary key, c_name varchar(32), c_age int check(c_age>=18) ); -- insert mysql> insert into t_check values(null,'column_check_001',18); Query OK, 1 row affected (0.04 sec) mysql> insert into t_check values(null,'column_check_002',17); Query OK, 1 row affected (0.00 sec) mysql> insert into t_check values(null,'column_check_003',30); Query OK, 1 row affected (0.00 sec) mysql> commit; Query OK, 0 rows affected (0.00 sec) mysql> select * from t_check; +----+------------------+-------+ | id | c_name | c_age | +----+------------------+-------+ | 1 | column_check_001 | 18 | | 2 | column_check_002 | 17 | | 3 | column_check_003 | 30 | +----+------------------+-------+ 3 rows in set (0.00 sec)
MySQL8.0.X check是有效的，做检查CHECK ,强制CHECK
在这个示例中，我们创建了一个名为 t_check 的表，其中有一个c_age 列。CHECK约束确保c_age 列的值在大于等于18。
mysql> create table t_check -> ( id int not null primary key, -> c_name varchar(32), -> c_age int check(c_age>=18) -> ); Query OK, 0 rows affected (0.15 sec) mysql> insert into t_check values(null,'column_check_001',18); Query OK, 1 row affected (0.01 sec)
mysql> insert into t_check values(null,'column_check_002',17); ERROR 3819 (HY000): Check constraint 't_check_chk_1' is violated.
mysql> insert into t_check values(null,'column_check_003',30); Query OK, 1 row affected (0.00 sec) mysql> commit; Query OK, 0 rows affected (0.02 sec) mysql> select * from t_check; +----+------------------+-------+ | id | c_name | c_age | +----+------------------+-------+ | 1 | column_check_001 | 18 | | 3 | column_check_003 | 30 | +----+------------------+-------+ 2 rows in set (0.00 sec)
create table t_check2 ( id int not null primary key, c_name varchar(32), c_age int ); alter table t_check2 add constraint check(c_age>=18); insert into t_check2 values(null,'column_check_001',18); insert into t_check2 values(null,'column_check_002',17); insert into t_check2 values(null,'column_check_003',30); commit; select * from t_check2;
同上用例测试 MySQL8.0 ,check有效检查并强制约束，执行效果如下
mysql> insert into t_check2 values(null,'column_check_002',17); ERROR 3819 (HY000): Check constraint 't_check2_chk_1' is violated.
SELECT * FROM information_schema.CHECK_CONSTRAINTS ;
mysql> SELECT * FROM information_schema.CHECK_CONSTRAINTS ; +--------------------+-------------------+-----------------+-----------------+ | CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME | CHECK_CLAUSE | +--------------------+-------------------+-----------------+-----------------+ | def | db01 | t_check2_chk_1 | (`c_age` >= 18) | +--------------------+-------------------+-----------------+-----------------+
select * from information_schema.TABLE_CONSTRAINTS
where CONSTRAINT_SCHEMA=‘superdb’
and TABLE_NAME=‘t_check2’;
mysql> select * from information_schema.TABLE_CONSTRAINTS
-> where CONSTRAINT_SCHEMA='superdb'
-> and TABLE_NAME='t_check2';
+--------------------+-------------------+-----------------+--------------+------------+-----------------+----------+
| CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME | TABLE_SCHEMA | TABLE_NAME | CONSTRAINT_TYPE | ENFORCED |
+--------------------+-------------------+-----------------+--------------+------------+-----------------+----------+
| def | superdb | PRIMARY | superdb | t_check2 | PRIMARY KEY | YES |
| def | superdb | t_check2_chk_1 | superdb | t_check2 | CHECK | YES |
+--------------------+-------------------+-----------------+--------------+------------+-----------------+----------+
2 rows in set (0.00 sec)
select * from information_schema.KEY_COLUMN_USAGE kcu
where CONSTRAINT_SCHEMA=‘superdb’
and TABLE_NAME=‘t_check’;
mysql> select * from information_schema.KEY_COLUMN_USAGE kcu
-> where CONSTRAINT_SCHEMA='superdb'
-> and TABLE_NAME='t_check';
+--------------------+-------------------+-----------------+---------------+--------------+------------+-------------+------------------+-------------------------------+-------------------------+-----------------------+------------------------+
| CONSTRAINT_CATALOG | CONSTRAINT_SCHEMA | CONSTRAINT_NAME | TABLE_CATALOG | TABLE_SCHEMA | TABLE_NAME | COLUMN_NAME | ORDINAL_POSITION | POSITION_IN_UNIQUE_CONSTRAINT | REFERENCED_TABLE_SCHEMA | REFERENCED_TABLE_NAME | REFERENCED_COLUMN_NAME |
+--------------------+-------------------+-----------------+---------------+--------------+------------+-------------+------------------+-------------------------------+-------------------------+-----------------------+------------------------+
| def | superdb | PRIMARY | def | superdb | t_check | id | 1 | NULL | NULL | NULL | NULL |
+--------------------+-------------------+-----------------+---------------+--------------+------------+-------------+------------------+-------------------------------+-------------------------+---
select * from information_schema.REFERENTIAL_CONSTRAINTS rc
where CONSTRAINT_SCHEMA=‘superdb’
and TABLE_NAME=‘t_check’;
mysql> select * from information_schema.REFERENTIAL_CONSTRAINTS rc
-> where CONSTRAINT_SCHEMA='superdb'
-> and TABLE_NAME='t_check';
Empty set (0.00 sec)
-- 因没有外键约束,查出为空
要删除一个 CHECK 约束，你需要知道它的名字（如果在创建时指定了的话）。但如果你没有指定名字，你可能需要删除整个表并重新创建它，或者使用其他方法（如触发器）来模拟 CHECK 约束的行为。
mysql> alter table t_check2 drop constraint t_check2_chk_1; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0
再次插入之前不满足条件的数据，则执行成功
mysql> insert into t_check2 values(null,'column_check_002',17); Query OK, 1 row affected (0.00 sec) mysql> commit; Query OK, 0 rows affected (0.00 sec) mysql> select * from t_check2; +----+------------------+-------+ | id | c_name | c_age | +----+------------------+-------+ | 1 | column_check_001 | 18 | | 2 | column_check_002 | 17 | | 3 | column_check_003 | 30 | +----+------------------+-------+ 3 rows in set (0.00 sec)
你也可以在多个列上使用 CHECK 约束，例如：
CREATE TABLE t_students (
id INT AUTO_INCREMENT PRIMARY KEY,
age INT,
grade CHAR(1),
CHECK (age >= 0 AND age <= 100),
CHECK (grade IN ('A', 'B', 'C', 'D', 'F'))
);
在这个示例中，我们添加了一个额外的 CHECK 约束来确保 grade 列的值是 ‘A’、‘B’、‘C’、‘D’ 或 ‘F’ 中的一个。
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