--这样吗？
----------------------------
-- Author  :fredrickhu(小F，向高手学习)
-- Date    :2012-07-17 14:56:27
-- Version:
--      Microsoft SQL Server 2008 R2 (RTM) - 10.50.1617.0 (Intel X86) 
--    Apr 22 2011 11:57:00 
--    Copyright (c) Microsoft Corporation
--    Enterprise Edition on Windows NT 6.1 <X64> (Build 7600: ) (WOW64)
--
----------------------------
--> 测试数据：[Tb1]
if object_id('[Tb1]') is not null drop table [Tb1]
go 
create table [Tb1]([code] varchar(3),[number] int,[datetime] datetime)
insert [Tb1]
select '001',2,'2012-3-2' union all
select '001',5,'2012-3-3' union all
select '002',3,'2012-3-4' union all
select '001',5,'2012-3-4'
--> 测试数据：[Tb2]
if object_id('[Tb2]') is not null drop table [Tb2]
go 
create table [Tb2]([code] varchar(3),[datetime] datetime)
insert [Tb2]
select '001','2012-3-3' union all
select '002','2012-3-6' union all
select '001','2012-3-2'
--------------开始查询--------------------------
select
  a.code,sum(a.number) as number
from
  tb1 a join tb2 b
on
  a.code=b.code 
where
  b.datetime=(select max(datetime) from tb2 where code=b.code)
and
  a.datetime>b.datetime
group by 
  a.code

----------------结果----------------------------
/* code number
---- -----------
001  5

(1 行受影响)
*/