求优化一条Select语句！谢谢

[soundofsea]

select (Case When Module='3873' THEN 
		(select DoorName from NaBon_DoorInfo WHERE DoorId = a.Form and ReadId = a.Intro)
	ELSE a.Form End) as Form, 

	(Case When Module='3873' THEN
		(select Impact from NaBon_DoorInfo WHERE DoorId = a.Form and ReadId = a.Intro)
	ELSE a.Intro End) as Intro

FROM dbo.Warder a

因为当Module不等'3873'时，SQL命令会出现错误，所以只有在特字值也才能查询 NaBon_DoorInfo表资料
我尝试了
FROM dbo.Warder a LEFT OUTER JOIN NaBon_DoorInfo d ON a.Module='3873' AND d.DoorId = a.Form AND d.ReadId = a.Intro
当Module不等'3873'时，也会报错。

我想将上边的代码进行一下缩写，因为我感觉这样会读三个表，从而会影响运行的速度。
求一个更好的写法！

[guguda2008]

select (Case When Module='3873' THEN 
        (select TOP 1 DoorName from NaBon_DoorInfo WHERE DoorId = a.Form and ReadId = a.Intro)
    ELSE a.Form End) as Form, 

    (Case When Module='3873' THEN
        (select TOP 1 Impact from NaBon_DoorInfo WHERE DoorId = a.Form and ReadId = a.Intro)
    ELSE a.Intro End) as Intro

FROM dbo.Warder a

[Johnson]

select d.doorname as form,d.impact as intro
from dbo.warder a join nabon_doorinfo d on d.doorid = a.form and d.readid = a.intro
where a.module='3873'
union all
select a.form  as form,a.intro as intro
from dbo.warder a 
where a.module<>'3873'