create table som
(序号 int, 日期 varchar(12), 余额 int)

insert into som
select 1, '20120101', null union all 
select 2, '20120102', null union all 
select 3, '20120103', 10 union all
select 4, '20120104', null union all 
select 5, '20120105', null union all 
select 6, '20120106', 20 union all
select 7, '20120107', null union all 
select 8, '20120108', null union all 
select 9, '20120109', 30 union all
select 10, '20120110', null union all 
select 11, '20120111', null 

select * from som

序号          日期           余额
----------- ------------ -----------
1           20120101     NULL
2           20120102     NULL
3           20120103     10
4           20120104     NULL
5           20120105     NULL
6           20120106     20
7           20120107     NULL
8           20120108     NULL
9           20120109     30
10          20120110     NULL
11          20120111     null


update a
set a.余额=
isnull((select b.余额 from som b 
 where b.余额 is not null and b.序号=
 (select max(序号) from som c
  where c.余额 is not null and c.序号<a.序号)
),
(select top 1 余额 from som where 余额 is not null))
from som a
where a.余额 is null


select * from som

序号          日期           余额
----------- ------------ -----------
1           20120101     10
2           20120102     10
3           20120103     10
4           20120104     10
5           20120105     10
6           20120106     20
7           20120107     20
8           20120108     20
9           20120109     30
10          20120110     30
11          20120111     30

(11 row(s) affected)