使用触发器做简单编程

CREATE TABLE goods (
  gid int(11) DEFAULT NULL,
  name varchar(20) DEFAULT NULL,
  num smallint(6) DEFAULT NULL
);


CREATE TABLE ord (
  oid int(11) DEFAULT NULL,
  gid int(11) DEFAULT NULL,
  much smallint(6) DEFAULT NULL
);

插入数据：
INSERT INTO goods VALUES (‘1‘,‘cat‘,‘32‘), (‘2‘,‘dog‘,‘65‘), (‘3‘,‘pig‘,‘21‘);

create trigger t
before
insert
on ord
for each row

begin

declare
rnum int;

select num into rnum from goods where gid = new.gid;

if new.much > rnum then
	set new.much = rnum;
end if;

update goods set num = num - new.much where gid=new.gid;
end