小白询问sql执行效率问题

[程爱动漫]

--1
declare @n int
select @n=isnull((select count(DingdanID) from TDingDan where DingDanID=@DingDanID),0)
if(@n<1) delete from TDingDanMingXi where DingDanID=@DingDanID

--2
delete from TDingDanMingXi where not exists(select DingDanID from TDingDan where DingDanID=@DingDanID) and DingDanID=@DingDanID

--3
select DingdanID from TDingDan where DingDanID=@DingDanID
--如果不存在记录执行
delete from TDingDanMingXi where DingDanID=@DingDanID

如上述，三种写法可以满足同一需求，请大佬指点迷津，那种写法比较好一些