Mysql 如何定义外键约束

我有三张mysql表。表已经创建

Requests -  request_id, request_message, user_id
Responses - response_id, response_message, user_id
users -     user_id, user_name

现在我想在上面定义外键约束，比如

1. If user_id is not present in Users table, and someone is inserting the data in Requests or Responses for that user_id -- then error
2. If request_id is not present in Requests table, then if someone is inserting in responses table for that request_id -- then error 

3. If someone deletes an user_id, all associated requests and responses with that user_id should be deleted automatically.
4. If someone deletes an request_id, all the associated responses with it, should be deleted automatically.    

如果我遗漏了什么，请告诉我

如何实现这一功能

谢谢

---

Devesh

以下是创建表的完整sql：

CREATE TABLE IF NOT EXISTS `reponses` (
  `response_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `response_message` varchar(45) DEFAULT NULL,
  `user_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`response_id`,`user_id`),
  KEY `fk_reponses_users1` (`user_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8mb4 AUTO_INCREMENT=3 ;


CREATE TABLE IF NOT EXISTS `requests` (
  `request_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `request_message` varchar(45) DEFAULT NULL,
  `user_id` int(10) unsigned NOT NULL,
  PRIMARY KEY (`request_id`,`user_id`),
  KEY `fk_requests_users` (`user_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8mb4 AUTO_INCREMENT=3 ;


CREATE TABLE IF NOT EXISTS `users` (
  `user_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
  `user_name` varchar(45) DEFAULT NULL,
  PRIMARY KEY (`user_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8mb4 AUTO_INCREMENT=3 ;


ALTER TABLE `reponses`
  ADD CONSTRAINT `reponses_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`user_id`) ON DELETE CASCADE ON UPDATE NO ACTION;

ALTER TABLE `requests`
  ADD CONSTRAINT `requests_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`user_id`) ON DELETE CASCADE ON UPDATE NO ACTION;

允许您删除与用户相关的记录的选项是delete CASCADE上的

。默认情况下，MySql设置了
NO ACTION
，它指的是
RESTRICT
，当父记录有相关对象时，不允许删除父记录。我想你没有提到回复和请求之间的关系，但你应该明白；）