MySQL 外键约束

最新推荐文章于 2025-07-29 16:16:45 发布

转载最新推荐文章于 2025-07-29 16:16:45 发布 · 118 阅读

本文详细介绍了MySQL中外键约束的不同类型及其应用场景，包括默认约束、CASCADE级联删除更新、SET NULL设置为空等，并通过具体实例展示了如何应用这些约束。

-- 创建测试主表. ID 是主键.
CREATE TABLE test_main (
id      INT,
value   VARCHAR(10),
PRIMARY KEY(id)
);

-- 创建测试子表.
CREATE TABLE test_sub (
id      INT,
main_id INT,
value   VARCHAR(10),
PRIMARY KEY(id)
);

-- 插入测试主表数据.
INSERT INTO test_main(id, value) VALUES (1, 'ONE');
INSERT INTO test_main(id, value) VALUES (2, 'TWO');

-- 插入测试子表数据.
INSERT INTO test_sub(id, main_id, value) VALUES (1, 1, 'ONEONE');
INSERT INTO test_sub(id, main_id, value) VALUES (2, 2, 'TWOTWO');

默认外键约束方式
mysql> ALTER TABLE test_sub
   ->   ADD CONSTRAINT main_id_cons
   ->   FOREIGN KEY (main_id)
   ->   REFERENCES test_main(id);
   -> //
Query OK, 2 rows affected (0.17 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> DELETE FROM
   ->   test_main
   -> WHERE
   ->   id = 1;
   -> //
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constrai
nt fails (`test`.`test_sub`, CONSTRAINT `main_id_cons` FOREIGN KEY (`main_id`) R
EFERENCES `test_main` (`id`))

MySQL使用下面这个语句删除外键约束
ALTER TABLE test_sub DROP FOREIGN KEY main_id_cons;

DELETE CASCADE 方式
mysql> ALTER TABLE test_sub
   ->   ADD CONSTRAINT main_id_cons
   ->   FOREIGN KEY (main_id)
   ->   REFERENCES test_main(id)
   ->   ON DELETE CASCADE//
Query OK, 2 rows affected (0.16 sec)
Records: 2 Duplicates: 0 Warnings: 0

mysql> DELETE FROM
   ->   test_main
   -> WHERE
   ->   id = 1;
   -> //
Query OK, 1 row affected (0.02 sec)

mysql> SELECT
   ->   *
   -> FROM
   ->   test_sub;
   -> //
+----+---------+--------+
| id | main_id | value |
+----+---------+--------+
| 2 |       2 | TWOTWO |
+----+---------+--------+
1 row in set (0.00 sec)

MySQL使用下面这个语句删除外键约束
ALTER TABLE test_sub DROP FOREIGN KEY main_id_cons;

UPDATE CASCADE方式
mysql> ALTER TABLE test_sub
   ->   ADD CONSTRAINT main_id_cons
   ->   FOREIGN KEY (main_id)
   ->   REFERENCES test_main(id)
   ->   ON UPDATE CASCADE;
   -> //
Query OK, 1 row affected (0.14 sec)
Records: 1 Duplicates: 0 Warnings: 0

mysql> UPDATE test_main SET id = 5 where id = 2
   -> //
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0

mysql> select * from test_sub//
+----+---------+--------+
| id | main_id | value |
+----+---------+--------+
| 2 |       5 | TWOTWO |
+----+---------+--------+
1 row in set (0.00 sec)

MySQL使用下面这个语句删除外键约束
ALTER TABLE test_sub DROP FOREIGN KEY main_id_cons;

SET NULL方式
mysql> ALTER TABLE test_sub
   ->   ADD CONSTRAINT main_id_cons
   ->   FOREIGN KEY (main_id)
   ->   REFERENCES test_main(id)
   ->   ON DELETE SET NULL;
   -> //
Query OK, 1 row affected (0.13 sec)
Records: 1 Duplicates: 0 Warnings: 0

mysql> DELETE FROM
   ->   test_main
   -> WHERE
   ->   id = 5;
   -> //
Query OK, 1 row affected (0.05 sec)

mysql>
mysql> SELECT
   ->   *
   -> FROM
   ->   test_sub;
   -> //
+----+---------+--------+
| id | main_id | value |
+----+---------+--------+
| 2 |    NULL | TWOTWO |
+----+---------+--------+
1 row in set (0.00 sec)