MYSQL选择共同的朋友

Question

我的'friends'表包含以下列:id,userID,userID2,state

userID和userID2没有特定的顺序放入数据库.

我目前使用此查询来查找用户朋友:

$quer = mysql_query("
SELECT CASE WHEN userID=$id THEN userID2 ELSE userID END AS friendID 
FROM friends WHERE userID=$id OR userID2=$id");

我试过这个但它不起作用:

SELECT 
CASE WHEN userID=$id OR userID=$session THEN userID2 
ELSE userID END AS friendID  
FROM friends WHERE (userID=$session OR userID2=$session) 
AND (userID=$id OR userID2=$id) AND friendID!=$session

如果friendID = $ session(我已经添加到我的第二个查询中)它也不应该返回该行

编辑:我想返回$ id和$ session共同的friendID行.我不确定为什么它不起作用.

Answer 1

解释后(实际上阅读"相互"部分):

SELECT a.friendID
FROM
      ( SELECT CASE WHEN userID = $id
                      THEN userID2 
                      ELSE userID 
               END AS friendID 
        FROM friends 
        WHERE userID = $id OR userID2 = $id
      ) a
  JOIN
      ( SELECT CASE WHEN userID = $session
                      THEN userID2 
                      ELSE userID 
               END AS friendID 
        FROM friends 
        WHERE userID = $session OR userID2 = $session
      ) b
    ON b.friendID = a.friendID