Question

我正在使用SQL Server。我有3张桌子：

tblUser
tblAudit
tblEnum

tblAudit有4列：

nUserId，nID：这两列指向tblUser

vFromValue，vToValue：这两列指向tblEnum

现在，我需要根据tblAudit表中的ID显示tblUser和tblEnum中的实际值。我写的查询看起来像这样：

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)

因为对于某些行，Audit表中的值对于列vFromValue和vToValue为空，所以这些值不是NULL，而是前一个值。如何通过更改上述查询来完整显示空值？请帮忙。

Answer 1

如果我理解你的问题 - 尝试对你的最后两行使用LEFT JOIN（并删除OR之后的条件）。

Answer 2

由于tblAudit.vFromValue和tblAudit.vToValue有NULL，请尝试以下操作：

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

或者这个：

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

在SQL Server联接中显示NULL值

2 个答案: