MySQL assertion-like constraint

问题

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

I got this table:

   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`) 
   ) 

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.

How can I manage it without assertions?

Thanks.

回答1:

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 ;

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.

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

来源：https://stackoverflow.com/questions/8839540/mysql-assertion-like-constraint