MS Access：查詢以添加“如果”條件

Question

我有以下MS Access查詢：

SELECT a.*
FROM Version_Change AS a
RIGHT JOIN
  (SELECT VERSION,
          min(ChangeDate) AS minimumdate,
          ObjectID
   FROM Version_Change
   GROUP BY ObjectID,
            VERSION) AS b ON(a.VERSION = b.VERSION)
AND (a.ChangeDate = b.minimumdate)
AND (a.ObjectID = b.ObjectID);

問題在於“ ChangeDate”列有時可以為空。因此，我希望查詢也顯示“ ChangeDate”為Null的字段。 如何在查詢中整合此條件？

Answer 1

如果一個版本和objectid的更改日期都為NULL則min(ChangeDate)也將為null，並且連接不會成功，因為a.ChangeDate = NULL將被評估為NULL，這是不正確的。

您可以這樣更改ON條件：

ON (a.version = b.version)
   AND
   (
     (a.ChangeDate = b.minimumdate)
     OR
     (a.ChangeDate IS NULL AND b.minimumdate IS NULL)
   )
   AND (a.ObjectID = b.ObjectID);

Answer 2

您可以對Nz使用舊的技巧：

AND (Nz(a.ChangeDate,b.minimumdate) = b.minimumdate)