MySQL 如何创建在插入前的Trigger

对于此，语法如下 −

delimiter //
create trigger yourTriggerName before insert on yourTableName
   FOR EACH ROW
   BEGIN

   yourStatement1
   .
   .
   N
END
//
delimiter ;

要理解上述语法，让我们创建一个表 −

mysql> create table DemoTable1919
   (
   Value int
   );
Query OK, 0 rows affected (0.00 sec)

以下是创建MySQL在插入前的Trigger的查询-

mysql> delimiter //
mysql> create trigger sumis60 before insert on DemoTable1919
   FOR EACH ROW
   BEGIN
   declare custom_message varchar(256);
        IF (SELECT sum(Value) from DemoTable1919) = 60 THEN
           set custom_message= "cannot insert because sum(Value) is greater than 60";
             signal sqlstate '13500'
             set message_text= custom_message;
         END IF;
     END
     //
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;

现在您可以使用插入命令在表中插入一些记录并检查触发器的工作情况-

mysql> insert into DemoTable1919 values(10);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1919 values(20);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1919 values(30);
Query OK, 1 row affected (0.00 sec)
mysql> insert into DemoTable1919 values(40);
ERROR 1644 (13500): cannot insert because sum(Value) is greater than 60

阅读更多：MySQL 教程