MySQL中的一对一关系和级联删除

Question

我有两个表：用户和管理员。

+-------------------------+
| TABLE : user
+-------------------------+
| id: PRIMARY_KEY
| username
| password
+-------------------------+

+-------------------------+
| TABLE: admin
+-------------------------+
| user_id: FOREIGN_KEY
| e-mail
+-------------------------+

管理员也是用户，因此，两个表的关系是 1:1。当从 User 表中删除一行时，它也会从表 Admin 中删除（在 admin.user_id 设置关系 ON DELETE CASCADE），但是如果我从表 Admin 中删除一行怎么办？然后它不会从 User 表中删除，这会在数据库中造成混乱。这是任何解决方案吗？如何实现双方的关系？

Answer 1

给admin表添加相似的外键-

ALTER TABLE user
  ADD CONSTRAINT FK_user_admin_user_id FOREIGN KEY (id)
    REFERENCES admin(user_id) ON DELETE CASCADE ON UPDATE RESTRICT;

现在，您可以从user 或admin 表中删除行，相关记录将被删除。

使用 FOREIGN_KEY_CHECKS 变量添加新记录，例如-

SET FOREIGN_KEY_CHECKS = 0;

INSERT INTO user(id) VALUES(1);
INSERT INTO admin(user_id) VALUES(1);

SET FOREIGN_KEY_CHECKS = 1;

带输出的完整示例 -

CREATE TABLE user (
  id int(11) NOT NULL AUTO_INCREMENT,
  username varchar(255) DEFAULT NULL,
  password varchar(255) DEFAULT NULL,
  PRIMARY KEY (id),
  CONSTRAINT FK_user_admin_user_id FOREIGN KEY (id)
  REFERENCES admin (user_id) ON DELETE CASCADE ON UPDATE RESTRICT
)
ENGINE = INNODB;

CREATE TABLE admin (
  user_id int(11) NOT NULL AUTO_INCREMENT,
  `e-mail` varchar(255) DEFAULT NULL,
  PRIMARY KEY (user_id),
  CONSTRAINT FK_admin_user_id FOREIGN KEY (user_id)
  REFERENCES user (id) ON DELETE CASCADE ON UPDATE RESTRICT
)
ENGINE = INNODB;

SET FOREIGN_KEY_CHECKS = 0;

INSERT INTO user(id) VALUES(1);
INSERT INTO user(id) VALUES(2);
INSERT INTO user(id) VALUES(3);

INSERT INTO admin(user_id) VALUES(1);
INSERT INTO admin(user_id) VALUES(2);
INSERT INTO admin(user_id) VALUES(3);

SET FOREIGN_KEY_CHECKS = 1;

DELETE FROM user WHERE id = 1;
DELETE FROM admin WHERE user_id = 2;

SELECT * FROM user;
+----+----------+----------+
| id | username | password |
+----+----------+----------+
|  3 | NULL     | NULL     |
+----+----------+----------+

SELECT * FROM admin;
+---------+--------+
| user_id | e-mail |
+---------+--------+
|       3 | NULL   |
+---------+--------+