mysql如何处理外码约束

http://hi.baidu.com/wangzhiqing999/item/e74b2be28be445a3c00d7528

MySQL 外键约束

-- 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)