有什么方法可以在（1,2）（2,1）友谊场景中找到共同的朋友？

Question

I'm developing a website wherein when a person adds another person as a friend, 2 entries are created. Let's say uid 1 adds uid 2 as a friend, the following rows are created in MySQL.

activity_id   uid1 uid2
     1         1     2
     2         2     1

uid 2 becomes friends with uid 3 :

activity_id   uid1 uid2
     1         1     2
     2         2     1
     3         2     3
     4         3     2

What are the ways to get the mutual friends between uid 1 and uid 2 ? I'm using php and MySQL with no PDO experience (for now).

[EDIT] Okay, so I decided to organize/normalize the tables. The relationship only generates 1 row now. which makes the table:

activity_id   uid1 uid2
     1         1     2
     2         2     3

Answer 1

Assuming you have called your table 'friends'.

This query will find common friends for users 1 and 3.

SELECT a.uid2
  FROM friends a
    INNER JOIN friends b
       ON a.uid2 = b.uid2
  WHERE a.uid1 = 1
   AND b.uid1 = 3

Don't you think it might be a bit presumptuous to make 2 rows though? Maybe uid 2 doesn't really like uid 1 ? (OTY).

Answer 2

Looking at the table format from your other question , this should do what you want;

SELECT name FROM users u
JOIN friends f1
  ON u.uid = f1.uid OR u.uid = f1.fid
JOIN friends f2
  ON u.uid = f2.uid OR u.uid = f2.fid
WHERE (f1.uid=1 OR f1.fid=1) AND (f2.uid=3 OR f2.fid=3) 
  AND u.uid<>1 AND u.uid<>3;

Demo here .

Answer 3

To find all common friends of uid 1 and uid 2 :

SELECT t1.uid2
FROM   tbl t1
JOIN   tbl t2 USING (uid2)
WHERE  t1.uid1 = 1
AND    t2.uid1 = 2;

There is one special case, though: this query does not show whether 1 & 2 are friends!

BTW, your design seems a bit redundant. It should work with just one entry per friendship. (This query would have to be adapted for that, though.)

Basically this is another case of relational division . There are many different ways to skin this cat. Find quite under this related question - including performance testing and more information.

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Query for updated question

Now there is only 1 row per friendship - a user can pop up in either column:

SELECT uid
FROM (
    SELECT uid2 AS uid FROM tbl WHERE uid1 = 1
    UNION ALL
    SELECT uid1        FROM tbl WHERE uid2 = 1
    ) u1
JOIN (
    SELECT uid2 AS uid FROM tbl WHERE uid1 = 2
    UNION ALL
    SELECT uid1        FROM tbl WHERE uid2 = 2
    ) u2 USING (uid)

Answer 4

$query="SELECT distinct(uid2) FROM `test` where uid1=$uid union select distinct(uid1) from test where uid2=$uid";