sql高手请进~~~（exists和赋值的问题）
declare   @ID   bigint
if   exists   (select   @ID=ID
from   dbo.T_D_TA_WareHouseIn
where
MakerItemCD=@MakerItemCD   and
Amount=@Amount   and
MakeDate=@MakeDate   and
BoxNo=@BoxNo   and
UseFlag= '   ')
为什么加上@ID就不对呢，赋值的时候不能和exists共用还是怎么回事啊，帮帮忙啊

------解决方案--------------------
declare @ID bigint
select @ID=ID
from dbo.T_D_TA_WareHouseIn
where
MakerItemCD=@MakerItemCD and
Amount=@Amount and
MakeDate=@MakeDate and
BoxNo=@BoxNo and
UseFlag= ' ')
if @ID is null then print '不存在 ' else print cast(@ID as varchar)