两个表求和的数值相减

select dm1,SUM(je) from lsxhd where rq>='2015-03-01' and rq<='2015-03-31' and dm1='az' GROUP by dm1

select dm1,SUM(je) from lsthd where rq>='2015-03-01' and rq<='2015-03-31' and dm1='az' GROUP by dm1

lsxhd求和数值减lsthd求和数值，怎么写语句求大神帮忙！！！！！

举个例子：

 declare @num1 int , @num2 int
set @num1 = (select id from test where id = 17)
set @num2 = (select id from test where id = 10)
select @num1-@num2 as num

select a.num1-b.num2 from (select sum(je) num1 from lsxhd) a,(select sum(je) num2 from lsthd) b