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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
1、MySQL5.7.X CHECK 约束
MySQL5.7.X CHECK 约束是无效的，只做检查CHECK ，不强制CHECK
1.1、测试用例
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 wrapperConnection 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 secThreads: 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(1,'column_check_001',18);
Query OK, 1 row affected (0.04 sec)mysql> insert into t_check values(2,'column_check_002',17);
Query OK, 1 row affected (0.00 sec)mysql> insert into t_check values(3,'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)
2、MySQL8.X CHECK 约束
MySQL8.0.X check是有效的，做检查CHECK ,强制CHECK
2.1、创建表t_check，插入满足 CHECK 约束的数据
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(1,'column_check_001',18);
Query OK, 1 row affected (0.01 sec)2.2、尝试插入不满足 CHECK 约束的数据：
mysql> insert into t_check values(2,'column_check_002',17);
ERROR 3819 (HY000): Check constraint 't_check_chk_1' is violated.2.3、插入满足 CHECK 约束的数据
mysql> insert into t_check values(3,'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)
2.4、给已经创建好的表增加约束
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(1,'column_check_001',18);
insert into t_check2 values(2,'column_check_002',17);
insert into t_check2 values(3,'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(2,'column_check_002',17);
ERROR 3819 (HY000): Check constraint 't_check2_chk_1' is violated.
3、约束的相关查询及管理
3.1、8.0.X 视图information_schema.CHECK_CONSTRAINTS
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)
3.2、查看表的主键约束信息
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 |
+--------------------+-------------------+-----------------+---------------+--------------+------------+-------------+------------------+-------------------------------+-------------------------+---
3.3、查看表的外键约束信息
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
4、 删除 CHECK 约束：
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(2,'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)
5、在多个列上使用 CHECK 约束：
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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