在两个子查询中添加Case，在subtract上返回null

Question

我有两个表，都有qty列，我想从issued_donated_items中减去donated_items。它工作正常，直到issued_donated_items中没有记录，然后我的查询返回null

SELECT 
(
(SELECT Sum(quantity) AS tQty  FROM donated_items WHERE item_id=4)

-

(SELECT Sum(quantity_issued) AS issueQty  FROM issued_donated_items WHERE item_id=4)

)AS total

Answer 1

像这样使用 isnull（）：

SELECT 
(
(SELECT Sum(isnull(quantity,0)) AS tQty  FROM donated_items WHERE item_id=4)

-

(SELECT Sum(isnull(quantity_issued,0)) AS issueQty  FROM issued_donated_items WHERE item_id=4)

)AS total

对于ANSI标准SQL，使用 coalesce（）而不是 isnull（）。

Answer 2

我建议将子查询移到from子句并使用coalesce()：

SELECT (COALESCE(di.QTY, 0) - COALESCE(idi.issueQTY, 0)
       ) AS total
FROM (SELECT Sum(quantity) AS tQty FROM donated_items WHERE item_id = 4) di CROSS JOIN
     (SELECT Sum(quantity_issued) AS issueQty FROM issued_donated_items WHERE item_id = 4) idi;

例如，如果您想要查看这两个数字及其差异，则可以轻松地重复使用这些值。