您要这样做：

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

现在， FF 绑定到 FU ，然后绑定到 FB 。

Now, FF ties to FU, which then ties to FB. As they are all inner joined, you can use the law of association to understand that this acts like they are all linked together.

 FF = FU   FU = FB    Therefore FF = FB