类似于MySQL断言的约束 [英] MySQL assertion-like constraint

问题描述

我是MySQL的新手，我刚刚发现它不支持断言.

I'm a MySQL newbie, I just discovered that it doesn't support assertions.

我有这张桌子:

   CREATE TABLE `guest` (
   `ssn` varchar(16) NOT NULL,
   `name` varchar(200) NOT NULL,
   `surname` varchar(200) NOT NULL,
   `card_number` int(11) NOT NULL,
   PRIMARY KEY (`ssn`),
   KEY `card_number` (`card_number`),
   CONSTRAINT `guest_ibfk_1` FOREIGN KEY (`card_number`) REFERENCES `member` (`card_number`) 
   ) 

我需要的是一个成员最多可以邀请2位客人. 因此，在表guest中，我需要一个特定的card_number最多可以出现2次.

What I need is that a member can invite maximum 2 guests. So, in table guest I need that a specific card_number can appear maximum 2 times.

如何在没有断言的情况下进行管理?

How can I manage it without assertions?

谢谢.

推荐答案

这肯定在表'guest'上有BEFORE INSERT触发器的味道:

This definitly smells of a BEFORE INSERT trigger on the table 'guest':

DELIMITER $$
DROP TRIGGER IF EXISTS check_guest_count $$
CREATE TRIGGER check_guest_count BEFORE INSERT ON `guest`
  FOR EACH ROW BEGIN
    DECLARE numguests int DEFAULT 0;
    SELECT COUNT(*) INTO numguests FROM `guest` WHERE card_number=NEW.card_number;
    if numguests>=2 THEN
      SET NEW.card_number = NULL;
    END IF;
  END;
$$
DELIMITER ;

这基本上会查询当前的访客数，如果已经> = 2，则将card_number设置为NULL.由于card_number被声明为NOT NULL，因此将拒绝插入.

This basically looks up the current guest count, and if it is already >=2 sets card_number to NULL. Since card_number is declared NOT NULL, this will reject the insert.

在MySQL 5.1.41-3ubuntu12.10(Ubuntu Lucid)上测试并为我工作

Tested and works for me on MySQL 5.1.41-3ubuntu12.10 (Ubuntu Lucid)

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