create table xav
(C varchar(6), A int, B int)

insert into xav
select '扶手', 12, 10 union all   
select '扶手', 9, 9 union all  
select '扶手', 9, 9 union all  
select '扶手', 8, 8 union all  
select '扶手', 8, 7 union all  
select '扶手', 6, 6


with t as
(select row_number() over(order by (select 0)) rn,C,A,B from xav
)
select t1.C,t1.A,t1.B,isnull(cast(t1.B-t2.A as varchar),'') 'BA'
from t t1
left join t t2 on t1.rn=t2.rn-1

C      A           B           BA
------ ----------- ----------- ------------------------------
扶手     12          10          1
扶手     9           9           0
扶手     9           9           1
扶手     8           8           0
扶手     8           7           1
扶手     6           6           

(6 row(s) affected)

------解决方案--------------------

SQL code

drop table #t
create table #t(C nvarchar(4), A int, B int)
insert into #t
select N'扶手',    12,    10   union all
select N'扶手',    9,    9   union all
select N'扶手',    9,    9   union all
select N'扶手',    8,    8   union all
select N'扶手',    8,    7   union all
select N'扶手',    6,    6   

with ct as
(select *,ROW_NUMBER() Over ( order by A desc) rw from #t)
select t1.C,t1.A,t1.B, t1.B - t2.A as cal from ct t1 left join ct t2
on t1.rw  = t2.rw -1 

/*
C    A    B    cal
扶手    12    10    1
扶手    9    9    0
扶手    9    9    1
扶手    8    8    0
扶手    8    7    1
扶手    6    6    NULL
*/