Question

我正在加入桌子：OLD和NEW

表：OLD

Employee_Name   Status
Ann             Y
Bob             Y
Carol   
Doug            Y
Ellen   
Frank

表：新

Employee_Name   Status
Ann             Y
Bob 
Carol   
Ellen   
Frank

我可以通过此连接获得我想要的输出：

SELECT Old.Employee_Name AS [OldName]
  ,New.Employee_Name AS [NewName]
  ,Old.Status AS [OldStatus]
  ,New.Status AS [NewStatus]
FROM Old
   FULL OUTER JOIN New ON Old.Employee_Name = New.Employee_Name

这是：

OldName         NewName         OldStatus   NewStatus
Ann             Ann             Y           Y
Bob             Bob             Y   
Carol           Carol       
Doug                            Y   
Ellen           Ellen       
Frank           Frank

现在我想在查询中添加一个过滤器，以便它只保留新旧状态不同的行。

如果我尝试：

SELECT Old.Employee_Name AS [OldName]
  ,New.Employee_Name AS [NewName]
  ,Old.Status AS [OldStatus]
  ,New.Status AS [NewStatus]
FROM Old
   FULL OUTER JOIN New ON Old.Employee_Name = New.Employee_Name
WHERE Old.Status <> New.Status

......它没有给出正确的输出。

对于这个例子，我认为它会给出输出：

OldName         NewName         NewStatus   OldStatus
Bob             Bob             Y

......并且未能与Doug一起回归。

Answer 1

执行外部联接时，不匹配的行具有NULL个值。这就是您将条件从on子句移动到where子句的原因：

SELECT Old.Employee_Name AS [OldName],
       New.Employee_Name AS [NewName],
       Old.Status AS [OldStatus],
       New.Status AS [NewStatus]
FROM Old FULL OUTER JOIN
     New
     ON Old.Employee_Name = New.Employee_Name AND
        Old.Status <> New.Status;

Answer 2

如果SQL语句中的任一运算符为null，则该行将返回null（无行）。例如，如果a <> b a = 1和b = null a，则它始终不返回任何行，即使b和WHERE COALESCE(Old.Status, '') <> COALESCE(New.Status, '')不相等。< / p>

您需要将查询更改为以下内容：

{{1}}

我的OUTER JOIN查询的WHERE子句有什么问题？

2 个答案: