Question

I have a members table in which every site member is the unique id.
eg

id  firstname   secondname  emailaddress    country city    gender  

the second table is a friends table with the following structure

id  meid    friendid    date

what query would i use to get friend suggestions of a particular user based on mutual friends and sorted accordingly. before i was using php to loop thru and collect mutual friends but as the site grew, php started misbehaving and running out of memory.

This is the function i was using

//-----------------------------------------------    
function getFriendSuggestions($id)    
{
            $friendids=getFriendIdArray($id);  //returns list of your friends
            $networkids=getNetworkIdArray($id);//returns list of all members in your network(friends and their friends)
            $diff=array_merge(array(),array_diff($networkids,$friendids));
            $diff_mutual=array();
            $diff_mutual_total=array();
            for ($n=0;$n<count($diff);$n++)
            {
                $ff=getFriendIdArray($diff[$n]);
                $mf=array_merge(array(),array_intersect($ff,$friendids));
                $diff_mutual[]=$mf;
                $diff_mutual_total[]=count($mf);
            }
            $diff=array_merge(array(),$diff);
            $diff_mutual=array_merge(array(),$diff_mutual);
            $diff_mutual_total=array_merge(array(),$diff_mutual_total);    
            $w=$diff_mutual_total;
            arsort($w);
            $d=array();
            $dm=array();
            foreach ($w as $key => $value)
            {
                $d[]=$diff[$key];
                $dm[]=$diff_mutual[$key];
            }
            $cv=array($d,$dm);
            return $cv;
}

Answer 1

You want “friends of my friends, but who are not also my friends”.

SELECT
  me.id                               AS member_id,
  their_friends.friendid              AS suggested_friend_id,
  COUNT(*)                            AS friends_in_common
FROM
  members        AS me
INNER JOIN
  friends_map    AS my_friends
    ON my_friends.meid = me.id
INNER JOIN
  friends_map    AS their_friends
    ON their_friends.meid = my_friends.friendid
LEFT JOIN
  friends_map    AS friends_with_me
    ON  friends_with_me.meid     = their_friends.friendid
    AND friends_with_me.friendid = me.id
WHERE
  friends_with_me.meid IS NULL
GROUP BY
  me.id,
  their_friends.friendid