在mysql sql数据库中设置最大限制

Question

我有2张桌子。音乐会/门票

我想将Concert 1设置为最多100张票，而Concert 2最多设置200张票。所以喜欢将concert_id与最大数量的ticket_id相关联。

我如何在mysql中执行此操作？感谢

Answer 1

您可以编写触发器：

delimiter //
drop trigger if exists limitInsertsTrigger //
create trigger limitInsertsTrigger before insert on tickets
for each row
begin
    declare msg varchar(128);
    if  (select count(*) from tickets where concert_id = new.concert_id)> 
        (select max_tickets_number from concert where id=new.concert_id)
    then
        set msg = concat('MyTriggerError: Trying to insert too many tickets');
        signal sqlstate '45000' set message_text = msg;
    end if;
end //

请记住在Concert表中有一个max_tickets_number列。