SQL的减法`

求助SQL的减法``
select   ACBL   into   #aaa     from   RD0   where   ACIT= '2110101 'and
ACDT= '20070630 'and   BRNO= '0101 '  
select   ACBL   into   #bbb     from   RD0   where   ACIT= '21101 'and  
ACDT= '20070630 'and   BRNO= '0101 '


select     #bbb.ACBL   -   #aaa.ACBL


请问下为什么临时表的#bbb.ACBL不能减#aaa.ACBL呢``?该怎么写``

------解决方案--------------------
select #bbb.ACBL - #aaa.ACBL from #bbb,#aaa where 条件