数据库查询结果相减问题！
我想将两个查询结果相减，现在是其中一个查询结果为空，最后总结果就也为空了，我想让查询为空的字段查为0不知道是否可以？

SELECT U.[StoreNum]-I.[DetailNum] 
FROM 
(SELECT SUM(StoreNum) AS StoreNum FROM TEST1) AS U,
(SELECT SUM(DetailNum) AS DetailNum FROM TEST2) AS I

[解决办法]
select sum(StoreNum)StoreNum
from (
SELECT SUM(StoreNum)StoreNum AS StoreNum FROM TEST1
union all
SELECT -1*SUM(DetailNum)StoreNum AS DetailNum FROM TEST2)a
[解决办法]

SELECT isnull(U.[StoreNum],0) - isnull(I.[DetailNum] ,0)
FROM 
(SELECT SUM(StoreNum) AS StoreNum FROM TEST1) AS U,
(SELECT SUM(DetailNum) AS DetailNum FROM TEST2) AS I