我的OUTER JOIN查詢的WHERE子句有什麼問題？

Question

我連接表：OLD和NEW

表：OLD

Employee_Name Status 
Ann    Y 
Bob    Y 
Carol 
Doug   Y 
Ellen 
Frank 

表：NEW

Employee_Name Status 
Ann    Y 
Bob 
Carol 
Ellen 
Frank 

我能得到我想要與此連接的輸出：

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 

這是：

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

現在我想爲查詢添加一個過濾器，以便它只保留新舊狀態不同的行。

如果我嘗試：

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 

...它沒有提供正確的輸出。

在這個例子中，我認爲它會給輸出：

OldName   NewName   NewStatus OldStatus 
Bob    Bob    Y 

...並不能與道格返回行。

Answer 1

如果在SQL語句中的任一運算符是null，它將爲該行返回空（無行）。例如，如果您執行a <> b其中a = 1和b = null，它將始終返回沒有行，即使其a和b不等效。

您需要查詢更改爲類似這樣：

WHERE COALESCE(Old.Status, '') <> COALESCE(New.Status, '') 

Answer 2

當你做外連接時，不匹配的行具有NULL值。這就是爲什麼你從where條款移動條件的on條款：

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;