Question

I have three mysql tables. Tables are already created.

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

Now i want to define foreign key constraints on that, such that

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.    

If i am missing any thing please let me know.

How to achieve this functionality?

Thanks

Devesh

Answer 1

Here is full sql to create your tables:

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;

Option that allows you to delete records related to user is ON DELETE CASCADE. By default MySql sets NO ACTION which refers to RESTRICT and doesn’t allow parent record to be deleted while it has related objects. I think that you didn’t mention the relation between responses and requests but you should get the idea ;).