MySQL外键类型及应用场景总结

-- 创建父表
CREATE TABLE `users` (
  `user_id` INT NOT NULL AUTO_INCREMENT,
  `username` VARCHAR(255) NOT NULL,
  PRIMARY KEY (`user_id`)
);

-- 创建子表
CREATE TABLE `orders` (
  `order_id` INT NOT NULL AUTO_INCREMENT,
  `order_date` DATE NOT NULL,
  `user_id` INT,
  PRIMARY KEY (`order_id`)
);

-- 插入父表数据
INSERT INTO `users` (`username`) VALUES ('Alice');
INSERT INTO `users` (`username`) VALUES ('Bob');

-- 插入子表数据
INSERT INTO `orders` (`order_date`, `user_id`) VALUES ('2024-12-25', 1);
INSERT INTO `orders` (`order_date`, `user_id`) VALUES ('2024-12-26', 2);

-- 添加外键约束到现有的子表 `orders`
ALTER TABLE `orders`
ADD CONSTRAINT `fk_user_id`
FOREIGN KEY (`user_id`)
REFERENCES `users`(`user_id`)
ON DELETE RESTRICT
ON UPDATE RESTRICT;

-- 删除已被引用的外键
DELETE FROM `users` WHERE `user_id` = 1

-- 输出结果 --
> 1451 - Cannot delete or update a parent row: a foreign key constraint fails (`test`.`orders`, CONSTRAINT `fk_user_id` FOREIGN KEY (`user_id`) REFERENCES `users` (`user_id`) ON DELETE RESTRICT ON UPDATE RESTRICT)
> 查询时间: 0.013s


-- 修改已被引用的外键
UPDATE `users` SET `user_id` = 3 WHERE `user_id` = 1

-- 输出结果 --
> 1451 - Cannot delete or update a parent row: a foreign key constraint fails (`test`.`orders`, CONSTRAINT `fk_user_id` FOREIGN KEY (`user_id`) REFERENCES `users` (`user_id`) ON DELETE RESTRICT ON UPDATE RESTRICT)
> 查询时间: 0.009s

-- 添加外键约束到 `orders` 表，使用 CASCADE
ALTER TABLE `orders`
ADD CONSTRAINT `fk_user_id`
FOREIGN KEY (`user_id`)
REFERENCES `users`(`user_id`)
ON DELETE CASCADE
ON UPDATE CASCADE;

-- 删除已被引用的外键
DELETE FROM `users` WHERE `user_id` = 1

-- 输出结果 --
> Affected rows: 1
> 查询时间: 0.016s


-- 修改已被引用的外键
UPDATE `users` SET `user_id` = 3 WHERE `user_id` = 2

-- 输出结果 --
> Affected rows: 1
> 查询时间: 0.013s

-- 确保子表的外键列允许 NULL
ALTER TABLE `orders`
MODIFY COLUMN `user_id` INT NULL;

-- 添加外键约束到 `orders` 表，使用 SET NULL
ALTER TABLE `orders`
ADD CONSTRAINT `fk_user_id`
FOREIGN KEY (`user_id`)
REFERENCES `users`(`user_id`)
ON DELETE SET NULL
ON UPDATE SET NULL;

-- 删除已被引用的外键
DELETE FROM `users` WHERE `user_id` = 1

-- 输出结果 --
> Affected rows: 1
> 查询时间: 0.014s


-- 修改已被引用的外键
UPDATE `users` SET `user_id` = 3 WHERE `user_id` = 2

-- 输出结果 --
> Affected rows: 1
> 查询时间: 0.012s

-- 添加外键约束，使用 NO ACTION
ALTER TABLE `orders`
ADD CONSTRAINT `fk_user_id`
FOREIGN KEY (`user_id`)
REFERENCES `users`(`user_id`)
ON DELETE NO ACTION
ON UPDATE NO ACTION;

-- 删除已被引用的外键
DELETE FROM `users` WHERE `user_id` = 1

-- 输出结果 --
> 1451 - Cannot delete or update a parent row: a foreign key constraint fails (`test`.`orders`, CONSTRAINT `fk_user_id` FOREIGN KEY (`user_id`) REFERENCES `users` (`user_id`))
> 查询时间: 0.013s


-- 修改已被引甮的外键
UPDATE `users` SET `user_id` = 3 WHERE `user_id` = 2

-- 输出结果 --
> 1451 - Cannot delete or update a parent row: a foreign key constraint fails (`test`.`orders`, CONSTRAINT `fk_user_id` FOREIGN KEY (`user_id`) REFERENCES `users` (`user_id`))
> 查询时间: 0.025s