SQL中两个表按时间分客户汇总问题，求助！

如图，有两个表，一个是发货一个是退货，现在SQL要查询按月份及客户分类的汇总数据，怎么写？

谢谢各位解答，我现在已经发现不用转换日期了，如下面语句中的ABSID就是过账期间，直接引用即可。但是问题来了：我想用Order by对TD.LineSum这个字段按大小排序，应该怎么做？。

select sum(S1.WS) as WSS,S1.CrdName,S1.ABSID,S1.OwnerName
from

(Select sum(TD.LineSum) as WS,TM.CrdName,TM.ABSID,TM1.OwnerName
From SASal TM
LEFT JOIN SASalA TD on TD.DocEntry=TM.DocEntry
LEFT JOIN BPClt TM1 on TM1.CltID=TM.CrdID
Where TM.DocDate>=@DF and TM.DocDate<=@DT
Group by TM.CrdName,TM.ABSID,TM1.OwnerName

Union All

Select sum(-TD.LineSum) as WS,TM.CrdName,TM.ABSID,TM1.OwnerName
From SABak TM
LEFT JOIN SABakA TD on TD.DocEntry=TM.DocEntry
LEFT JOIN BPClt TM1 on TM1.CltID=TM.CrdID
Where TM.DocDate>=@DF and TM.DocDate<=@DT
Group by TM.CrdName,TM.ABSID,TM1.OwnerName)S1
Group by S1.CrdName,S1.ABSID,S1.OwnerName