删除最后一个匹配SQL上的记录

Question

I have 2 tables linked together as follows:

Owner table | owner_id = Primary Key
Car table | car_id = Primary Key | owner_id = Foreign Key

Basically, every time a new car record is added, it is assigned to an owner. An owner can have multiple cars, but each car can only belong to one owner.

Is it possible to set a constraint or something that would delete the owner from the owner table on the last "matching" record.

For instance, owner_1 owns 5 cars. After the last car that they own is deleted, then it also drops the owner record from the database.

Answer 1

I think an after delete trigger would do the job. Every time you delete a car record, you should query the same car table to see if there is another car with the same owner as the one you are deleting. something like this:

EDIT: Fixing Select Query :D

DELIMITER $$
CREATE TRIGGER delete_owners_delete AFTER DELETE ON Car FOR EACH ROW
BEGIN
   DECLARE iCounter integer;
   SELECT COUNT(owner_id) INTO @iCounter FROM car c WHERE c.owner_id = old.owner_id;
   IF (@iCounter < 1) THEN
      DELETE FROM Owner WHERE owner_id = old.owner_id;
   END IF;
END; 
$$    

Hope it helps. Cheers!