SET NOCOUNT ON
create TAble A (col1 float, col2 float)

insert A (col1)
select 1
union all select 2
union all select 3
union all select 4
union all select 5
GO

With Ac (rowID, col1, col2)
AS
(    Select row_number() over(order by Current_TimeStamp) as rowID, col1, col2
    From A
)

select (select sum(col1) from Ac where rowID <= Ac1.rowID) as col1,  
    CASE WHEN rowid= 1 THEN (select sum(col1)/2 from AC where rowID <=2)
        WHEN rowID = (select max(rowID) from Ac) THEN (select sum(col1)/2 from AC where
        rowID >= Ac1.rowID-1)
        ELSE (select sum(col1/3) from AC where rowID >= Ac1.rowID-1 AND rowID <= Ac1.rowID+1)
    END as col2
FROM Ac Ac1

drop table A

------解决方案--------------------
不是很理解楼主的意思

------解决方案--------------------
是计算值还是显示字符？是计算：
create table A(col1 int)
insert A select 3  
insert A select 2  
insert A select 5  
insert A select 1  
insert A select 4
-----------------------
select *,identity(int,1,1) Cid into #B from A--借用临时表
select col1,(select sum(col1) from #B b where b.cid<=a.cid)累计和,
(select avg(col1*1.0) from #B c where abs(c.cid-a.cid)<=1)前后平均数 from #B a  
------------------------------------
col1        累计和         前后平均数                                   
----------- ----------- ----------------------------------------  
3           3           2.500000
2           5           3.333333
5           10          2.666666
1           11          3.333333
4           15          2.500000

（所影响的行数为 5 行）