SQL自连接问题

Question

I've got 2 tables that I need to get data from...my users table looks like this:

Users Table
-------------
UserID
FirstName
LastName
WebLogin
WebPassword
Active

UserAlternates Table
---------------------
UserAlternateID
UserID
AlternateUserID

Users Table Data
------------------
1, John, Brown, jbrown, jbrown, true
2, Mark, Smith, msmith, msmith, true
3, Tim, Stone, tstone, tstone, true

UsersAlternate Table Data
--------------------------
1, 1, 2
2, 1, 3
3, 2, 1
4, 3, 2
5, 3, 1

The UserID refers back to the UserID in the Users table and so does the AlternateUserID. This is a case where our program can have users that are "alternates" to other users. So in the above example, if John Brown would have Mark & Tim as Alternates, and Mark would have John as an alternate while Time would have Mark and John as alternates. I'm drawing a blank on how to write the SQL to show the alternate users for a given userid. So if I passed in UserID = 1, it would return:

2, Mark, Smith
3, Tim, Stone

I tried this but it returns 2 rows of the same user data (in this case, 2 John Brown's):

CREATE      PROCEDURE [dbo].[GetUserAlternates]
@UserID int

 AS
SELECT u.FirstName, u.LastName, ua.AlternateUserID
FROM Users u
INNER JOIN UserAlternates ua ON u.UserID = ua.AlternateUserID
WHERE u.UserID = @UserID

Any ideas?

Answer 1

CREATE PROCEDURE dbo.GetUserAlternates
    @UserID INT
AS
BEGIN
    SET NOCOUNT ON;

    SELECT u.FirstName, u.LastName, u.UserID
      FROM dbo.Users AS u
      INNER JOIN dbo.UserAlternates AS au
      ON u.UserID = ua.AlternateUserID
    WHERE ua.UserID = @UserID; -- key difference here!
END
GO

Answer 2

How about something like

SELECT  u.*
FROM    UserAlternates ua   INNER JOIN
        Users u ON  ua.AlternateUserID = u.UserID
WHERE   ua.UserID = @UserID 

It does not seem from your request that you need to join to the Users table twice, as the UserAlternates table already contains the original UserID.

Answer 3

You've got the wrong table alias:

WHERE ua.UserID = @UserID

Note: ua not u.

Answer 4

SELECT ua.AlternateUserID, U2.FirstName, U2.Lastname
FROM Users u
  INNER JOIN UserAlternates ua ON u.UserID = ua.UserID
  INNER JOIN Users U2 on U2.UserID = UA.AlternateUserID
WHERE u.UserID = @UserID