MySQL 外键约束和触发器，级联删除

-- 创建测试主表. ID 是主键.
CREATE TABLE test_main (
id INT NOT NULL,
value VARCHAR(10),
PRIMARY KEY(id)
);

-- 创建测试子表.
CREATE TABLE test_sub (
id INT NOT NULL,
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');

通过 DELETE CASCADE 外键约束的处理机制来处理
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)

如果你非要使用触发器来处理， 那么下面是一个例子：

-- 创建测试主表. ID 是主键.
CREATE TABLE t_test_main (
id INT NOT NULL,
value VARCHAR(10),
PRIMARY KEY(id)
);

-- 创建测试子表.
CREATE TABLE t_test_sub (
id INT NOT NULL,
main_id INT ,
value VARCHAR(10),
PRIMARY KEY(id)
);

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

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

DELIMITER //

CREATE TRIGGER tr_t_test_main_Del
BEFORE DELETE ON t_test_main
FOR EACH ROW
BEGIN
DELETE FROM t_test_sub WHERE main_id = OLD.id;
END;
//

DELIMITER ;

mysql> delete from t_test_main where id = 1;
Query OK, 1 row affected (0.01 sec)

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