求一分组SQL

[clare2003]

select sum(gw)as AA from tup992004 where pay_yy='2004' and pay_mm='4'
group by Dept_no)

select dept_no,sum(gw) from tup992004 where pay_yy='2004' and pay_mm='3'
group by Dept_no
这里我能分组求出每个月的工资总额,但我想实现按部门分组,得到两个月的差值在一条语句中实现
dept_no,差额
------------
XXXXXXX,123
XXXXXX1,234

[clare2003]

up

[victorycyz]

select dept_no,
sum(case when pay_mm='4' then gw else (-1)*gw end) as 差值
from tup992004
where pay_yy='2004' and (pay_mm='3' or pay_mm='4')
group by Dept_no