SQL Server INNER JOIN具有多個關系的多個內部聯接

Question

我有以下查詢。 它工作正常但我需要從另一個名為FB的表中提取BUserName，該表具有與FU表中的UserID相關的UserID字段。 這可能嗎？

    SELECT TOP 100 
    FF.XID, 
    FF.YID, 
    FF.Title, 
    FF.FileName, 
    FF.FilePath, 
    FU.UserName as FUUserName, 
    FU.UserName as BUserName
    FROM FF 
    INNER JOIN FU ON FU.UserID = FF.UserID 

只是為了澄清。 我在FB表中沒有UserName列。 它確實有FB.UserID，它與FF.UserID有關，這是我想從中提取第二個UserName的地方。 因此，在這種關系中，我試圖從與FB表中的userID相關的FF.UserID表中拉下用戶名。 這有道理嗎？

Answer 1

你想要這樣的東西：

SELECT TOP 100 
FF.XID, 
FF.YID, 
FF.Title, 
FF.FileName, 
FF.FilePath, 
FU.UserName as FUUserName, 
FU.UserName as BUserName,
FB.BUserName as FB_BUserName
FROM FF 
    INNER JOIN FU ON FU.UserID = FF.UserID 
    INNER JOIN FB ON FB.UserID = FU.UserID

現在， FF與FU聯系，然后與FB聯系。 由於它們都是內在的聯系，你可以使用結社法來理解這種行為就像它們都是聯系在一起的。

 FF = FU   FU = FB    Therefore FF = FB

Answer 2

另外兩個例子

1-結果為null：

use my_DB
SELECT tbl_users.UserName
,Updated.UserName   /*Updated not in tbl*/

FROM [my_DB].[dbo].[my_tbl]
left outer join  tbl_users ON tbl_users.UserID = my_tbl.UserID
left outer join tbl_users Updated ON tbl_users.UserID = my_tbl.LasUpdatedUserID

2-沒有結果為null：

SELECT tbl_users.UserName
,Updated.UserName   /*Updated not in tbl*/
FROM [my_DB].[dbo].[my_tbl]
INNER JOIN  tbl_users ON tbl_users.UserID = my_tbl.UserID
INNER JOIN tbl_users Updated ON tbl_users.UserID = my_tbl.LasUpdatedUserID

**

Answer 3

像那樣？

SELECT TOP 100  
    FF.XID,  
    FF.YID,  
    FF.Title,  
    FF.FileName,  
    FF.FilePath,  
    FU.UserName as FUUserName,  
    FU.UserName as BUserName,
    FB.BUserName  
    FROM FF  
    INNER JOIN FU ON FU.UserID = FF.UserID  
    INNER JOIN FB ON FU.UserID = FB.UserID  

Answer 4

SELECT TOP 100 
FF.XID, 
FF.YID, 
FF.Title, 
FF.FileName, 
FF.FilePath, 
FU.UserName as FUUserName, 
FB.UserName as BUserName
FROM FF 
INNER JOIN FU ON FU.UserID = FF.UserID 
INNER JOIN FB ON FU.UserID = FB.UserID 

Answer 5

 SELECT TOP 100 
    FF.XID, 
    FF.YID, 
    FF.Title, 
    FF.FileName, 
    FF.FilePath, 
    FU.UserName as FUUserName, 
    FB.BUserName
    FROM FF 
    INNER JOIN FU ON FU.UserID = FF.UserID 
    INNER JOIN FB ON FB.UserID = FU.UserID