如何定義外鍵約束

Question

我有三個mysql表。表已經創建好了。

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

現在我想在定義外鍵約束，這樣

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.  

如果我錯過任何事，請讓我知道。

如何實現此功能？

感謝

Devesh

Answer 1

下面是完整的SQL來創建表：

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; 

選項，使您可以刪除與用戶記錄是ON DELETE CASCADE。默認情況下，MySql設置NO ACTION，它指的是RESTRICT，並且不允許父記錄在具有相關對象時被刪除。我認爲你沒有提到答覆和請求之間的關係，但你應該明白;）。