在SQL Server中顯示NULL值加入

Question

我正在使用SQL Server。我有3個表：

1. tblUser
2. tblAudit
3. tblEnum

的tblAudit得到了4列：

nUserId，的NID：這兩列指向tblUser

vFromValue，vToValue：這兩列指向tblEnum

現在，我需要根據tblAudit表中存在的ID顯示來自tblUser和tblEnum的實際值。 我寫的查詢看起來是這樣的：

SELECT A.aEventID, 
U.tName AS MakerChecker, 
U2.tName AS OnUser, 
VLE.tDisplayName AS FromValue, 
VLE2.tDisplayName AS ToValue, 
A.dChangeTime AS Timestamp 
FROM tblAudit A INNER JOIN tblUser U ON A.nUserId = U.aUserId 
INNER JOIN tblUser U2 ON A.nID = U2.aUserId 
INNER JOIN tblEnum VLE ON A.vFromValue = VLE.nIndex OR (A.vFromValue IS NULL) 
INNER JOIN tblEnum VLE2 ON A.vToValue = VLE2.nIndex OR (A.vToValue IS NULL) 

因爲在審覈表中的值是零對於一些行的列vFromValue和vToValue，這些值都不會再爲NULL，但以前的值。 如何通過更改上述查詢來完整顯示空值？請幫忙。

Answer 1

由於tblAudit.vFromValue和tblAudit.vToValue有NULL S，試試這個：

SELECT A.aEventID, 
     U.tName AS MakerChecker, 
     U2.tName AS OnUser, 
     VLE.tDisplayName AS FromValue, 
     VLE2.tDisplayName AS ToValue, 
     A.dChangeTime AS Timestamp 
FROM 
    (SELECT aEventID, nUserId, nID, dChangeTime 
      , COALESCE(vFromValue, -9999) AS vFromValue --- some non-existent 
      , COALESCE(vToValue, -999) AS vToValue  --- value 
     FROM tblAudit 
    ) AS A 
    INNER JOIN tblUser U 
    ON A.nUserId = U.aUserId 
    INNER JOIN tblUser U2 
    ON A.nID = U2.aUserId 
    LEFT JOIN tblEnum VLE 
    ON A.vFromValue = VLE.nIndex 
    LEFT JOIN tblEnum VLE2 
    ON A.vToValue = VLE2.nIndex 

或本：

SELECT A.aEventID, 
     U.tName AS MakerChecker, 
     U2.tName AS OnUser, 
     (SELECT VLE.tDisplayName 
     FROM tblEnum VLE 
     WHERE A.vFromValue = VLE.nIndex 
     ) AS FromValue, 
     (SELECT VLE2.tDisplayName 
     FROM tblEnum VLE2 
     WHERE A.vToValue = VLE.nIndex 
     ) AS ToValue, 
     A.dChangeTime AS Timestamp 
FROM tblAudit AS A 
    INNER JOIN tblUser U 
    ON A.nUserId = U.aUserId 
    INNER JOIN tblUser U2 
    ON A.nID = U2.aUserId 

Answer 2

如果我理解你的問題 - 嘗試對最後兩行使用LEFT JOIN（並在OR之後刪除條件）。