MySQL的：INNER JOIN

Question

我有一個包含用戶ID &他的朋友們想要開個表：

---------------------------------------------- 
UserFriendsId | UserId | FriendId 
---------------------------------------------- 
    1     1    2 
---------------------------------------------- 
    2     1    3 
---------------------------------------------- 
    3     2    1 
---------------------------------------------- 
    4     2    3 
---------------------------------------------- 

此表的數據顯示，用戶1 &用戶-2是朋友&他們也有frndship與用戶3 。 現在我想找到其中用戶ID 1個&用戶ID 2普通朋友（個），如： 在森坦斯我的查詢：用戶1 &用戶2有1個普通朋友FriendId 3.

爲此，我使用SQL查詢INNER JOIN：

SELECT t1.* 
    FROM userfriends t1 
INNER JOIN userfriends t2 
    ON t1.FriendId = t2.FriendId 
WHERE t1.UserId = 2 

，但不能退貨要求的結果..

Answer 1

select * 
    from MyTable as A 
inner join MyTable as B 
    on  (A.UserID = 1 and B.UserID = 2) 
     and (A.FriendID = B.FriendID) 

編輯

Answer 2

請試試這個

select u1.FriendId 
from userfriends u1 
inner join userfriends u2 on u1.FriendId = u2.FriendId 
where u1.UserId = 1 and u2.UserId = 2 

Answer 3

嘗試此查詢（非常類似於TH別人貼）

SELECT t1.UserID,T2.userID,T1.FriendID 
    FROM userfriends t1 
    JOIN userfriends t2 
    ON  (t1.FriendId = t2.FriendId) 
     and (T1.userID <>T2.userID) 
WHERE t1.UserId = 2 
    and T2.userId=1 

要注意的是這個報告每友誼鏈的兩倍，一旦你刪除了絕對的ID。爲了避免這種情況，我要求T1.UserID比T2.userID小：

SELECT t1.UserID,T2.userID,T1.FriendID 
    FROM userfriends t1 
    JOIN userfriends t2 
    ON  (t1.FriendId = t2.FriendId) 
     and (T1.userID < T2.userID) 

Answer 4

SELECT table1.name, table2.salary 
    FROM employee AS table1 INNER JOIN info AS table2 ON table1.name = table2.name;