MySQL：在不使用Distinct的情况下将表与自身连接

Question

这是我的表结构

• fName 朋友的名字[Varchar]
• fUID 朋友的ID [Int]
• UID 用户ID [Int]

pk [fUID，UID]

我想结识2个人的共同朋友。 我试过了：

SELECT t1.fName,
       t1.fUID 
  FROM friends t1, 
       friends t2 
 WHERE t1.UID = X 
   AND t2.UID = X2

但它没有用:(我反复得到相同的结果。

一些样本数据

UID  Name  
----------
1    Mark  
2    Eduardo  
3    Dustin  

Friend Table

UID  fUID  fName  
------------------
1    2     Eduardo   
2    1     Mark  
3    1     Mark  
1    3     Dustin 

Answer 1

如果我对您的理解正确，则您有两个用户ID（X1和X2），并且您想查找与这两个人成为朋友的所有人。

SELECT f1.fUID, f1.fName
FROM friends f1,
friends f2
WHERE f1.UID = X1
      AND f2.UID = X2
      AND f1.fUID = f2.fUID

注意：这假定所有朋友关系在friends表中存储两次，每个方向一次。 如果没有，它将变得更加复杂。

Answer 2

尝试这个：

   SELECT DISTINCT t1.fName,t1.fUID 
     FROM friends t1, friends t2 
    WHERE t1.UID=<USER-ID-1>
      AND t2.UID=<USER-ID-2>
      AND t1.fUID = t2.fUID
      AND t1.UID <> t2.UID

测试数据设置：

CREATE TABLE Friends
(
    uID INT,
    fuID INT,
    fName VARCHAR(20)
)

INSERT INTO Friends
SELECT '1 ',' 2 ','Eduardo'
UNION
SELECT '2 ',' 1 ','Mark'
UNION
SELECT '3 ',' 1 ','Mark'
UNION
SELECT '1 ',' 3 ','Dustin' 


SELECT DISTINCT t1.fName,t1.fUID 
    FROM Friends t1, Friends t2 
 WHERE t1.UID=2
   AND t2.UID=3
     AND t1.fUID = t2.fUID

Answer 3

SELECT t1.fName, t1.fUID FROM friends t1 WHERE t1.UID=X1
Union
SELECT t1.fName, t1.fUID FROM friends t1 WHERE t1.UID=X2

您可以使用联合查询来获取共同的朋友。

Answer 4

select fName
from friends
where uid = FIRST_USER_ID
  and exists (
    select 1
    from friends sfriends
    where sfriends.fid = friends.fid
      and sfriends.uid = SECOND_USER_ID
);

Answer 5

SELECT
  f.fUID,
  u.Name
FROM (
  SELECT fUID
  FROM (
    SELECT
      CASE WHEN UID  IN (@X, @X2) THEN UID ELSE fUID END AS UID,
      CASE WHEN fUID IN (@X, @X2) THEN UID ELSE fUID END AS fUID
    FROM friends
    WHERE     (UID IN (@X, @X2) OR  fUID IN (@X, @X2))
      AND NOT (UID IN (@X, @X2) AND fUID IN (@X, @X2))
  ) s
  GROUP BY fUID
  HAVING COUNT(DISTINCT UID) = 2
) f
  INNER JOIN users u ON f.fUID = u.UID