mysql触发器问题

Question

看了数据库的触发器，就想，如果某个定单表插入的数量大于商品表，那么商品表应该是负数啊，应该加以限制。可是我采用before来限制时，则不能执行，谁帮我看下这串代码哪错了

create trigger tg1

before insert on ord

for each row

begin

if new.o_num > goods.g_num then

set new.o_num = goods.g_num

end if;

update goods set g_num = g_num - new.o_num where gid = new.gid;

end$