SQL 更新修改删除一个表，库存自动增减的写法

create trigger tri_asb
on asb for insert 
as 
begin
  declare @rk int
  declare @ck int
  declare @sid varchar(20)
  select @sid=商品代码 ,@rk =入库数量,@ck=出库数量 from inserted
  if not exists(select * from asb where 商品代码=@sid) 
    insert into bsb(商品代码,数量) values(@sid,@rk-@ck)
  else
    update bsb set 数量=数量+(@rk-@ck) where 商品代码=@sid
end