--原始数据：@T
declare @T table(帐号 varchar(3),起算日期 varchar(8),变动日期 varchar(8),余额 decimal(6,2))
insert @T
select '001',20070205,20070506,1000.00 union all
select '001',20070205,20070806,1200.00 union all
select '001',20070205,20071106,1400.00 union all
select '001',20070205,29999999,1400.00 union all
select '002',20070109,20070710,500.00 union all
select '002',20070109,29999999,500.00

declare @1 varchar(8),@2 varchar(8)
select @1 = '20070801',@2='20070901'

select 帐号=coalesce(a.帐号,b.帐号),
余额=(isnull(a.余额,b.余额)*datediff(day,isnull(a.变动日期,@1),isnull(b.变动日期,@2))+
isnull(b.余额,0)*datediff(day,isnull(b.变动日期,@1),@2))/datediff(day,@1,@2)
from
(select 帐号,变动日期=@1,余额 from @T a where 变动日期=(select max(变动日期) from @T where 帐号=a.帐号 and 变动日期<@1)) a
full join
(select 帐号,变动日期,余额 from @T where left(变动日期,6)=left(@1,6)) b
on a.帐号=b.帐号

/*
帐号    余额
001    1167.7419354838709
002    500.0000000000000
*/