多条单据，每个单据多条明细，循环时，判断出每个单据的开始

--create table ABC
--(
--    A int,
--    B int,
--    C int
--)
--insert ABC select '1','20',null
--insert ABC select '1','10',null
--insert ABC select '2','15',null
--insert ABC select '2','5',null

if exists(select * from sysobjects where id=object_id(N'tempdb..#ttt') )
drop table #ttt
DECLARE @sid INT,@sno INT,@preid int,@curid int,@ccid int,@ctotal int
select IDENTITY(int,1,1) as fid,* into #ttt from ABC order by A asc, B desc

SET @sno=@@rowcount
SET @sid=1
set @preid=0
set @ctotal=0
WHILE @sid<=@sno
BEGIN
select @curid=A from #ttt where fid=@sid
select @ccid=B from #ttt where fid=@sid
if @preid<>@curid
begin
set @ctotal=@ccid
set @preid=@curid
end
else
begin
set @ctotal=@ctotal-@ccid
end
update #ttt set c=@ctotal where fid=@sid
SET @sid=@sid+1
END
select * from #ttt
drop table #ttt