I'm building a social network, and I want my members to be able to easily find new friends. Just like in Facebook, I want to suggest them some people they may know by the number of mutual friends they have.
My PostgreSQL database structure for friendships is as following:
> PROFILES_FRIENDSHIPS
> ----------------------
> - fri_profile_from // int, Person who sent the friendship request
> - fri_profile_to // int, Person who received the friendship request
> - fri_accepted // tinyint, has the person accepted the friendship request?
This is the query I figured out in PostgreSQL for finding the number of mutual friends between 2 profiles (profile with ID 24, and profile with ID 26):
SELECT COUNT(*)
FROM profiles AS p
INNER JOIN (
SELECT (
CASE WHEN ( 26 = f.fri_profile_from ) THEN f.fri_profile_to
ELSE f.fri_profile_from END) AS fri_profile_from
FROM profiles_friendships AS f
WHERE 1 = 1
AND (f.fri_profile_to = 26 OR f.fri_profile_from = 26)
AND fri_accepted = 1)
AS f1
ON (f1.fri_profile_from = p.pro_id)
INNER JOIN (
SELECT (
CASE WHEN ( 24 = f.fri_profile_from ) THEN f.fri_profile_to
ELSE f.fri_profile_from END) AS fri_profile_from
FROM profiles_friendships AS f
WHERE 1 = 1
AND (f.fri_profile_to = 24 OR f.fri_profile_from = 24)
AND fri_accepted = 1)
AS f2
ON (f2.fri_profile_from = p.pro_id)
Now I have been trying to convert this query to make it find the profiles with the most mutual friends of me, but who are not friends with me. But without success... I also researched a lot of examples on this website, but most them are working with double records in the friendships table. Like if 24 is friends with 26, there are 2 records: (24, 26) and (26, 24). That makes them easier to join and to find mutual friends, but that's not how I want to build my database.
If someone could help me to get started on this query, I would be very grateful.
Step 1 Get everyone that isnt a friend
Select *
From profiles
where (from/to_friendship is not myID)
step 2 include a column with # of mutual friends and order by it
select *,
(select count(*) from [mutual friends query]) as NrOfMutualFriends)
From profiles
where (from/to_friendship is not myID)
Order by NrOfMutualFriends
Edit: Mutual Friends query:
Step 1 select all my friends and all his friends
select if(from = myId, to, from) as myfriendids
from PROFILES_FRIENDSHIPS where from = myid or to = myid
select if(from = hisId, to, from) as hisfriendids
from PROFILES_FRIENDSHIPS where from = hisId or to = hisId
Step 2 Combine these queries into 1
select count(*)
from
( select if(from = myId, to, from) as myfriendids
from PROFILES_FRIENDSHIPS where from = myid or to = myid) myfriends
inner join
( select if(from = hisId, to, from) as hisfriendids
from PROFILES_FRIENDSHIPS where from = hisId or to = hisId) hisfriends
on myfriendsids = hisfriendsids
WITH friends AS(
SELECT p.pro_id, CASE WHEN f.fri_profile_from = p.pro_id THEN f.fri_profile_to
ELSE f.fri_profile_from END AS friend_id
FROM profiles
)
SELECT f2.pro_id, count(*) as friend_count
FROM friends AS f1
JOIN friends AS f2
ON f1.friend_id=f2.friend_id
AND f1.pro_id != f2.pro_id
AND f1.pro_id != f2.friend_id
WHERE f1.pro_id = :user_id
GROUP BY f2.pro_id
ORDER BY friend_count;
you can easily create inline view in double records format:
with cte_friends(user_id, friend_id) as (
select
fri_profile_from, fri_profile_to
from PROFILES_FRIENDSHIPS
where fri_accepted = 1
union all -- or union if there could be duplicates
select
fri_profile_to, fri_profile_from
from PROFILES_FRIENDSHIPS
where fri_accepted = 1
)
select
f2.friend_id, count(distinct f2.user_id)
from cte_friends as f1
inner join cte_friends as f2 on f2.user_id = f1.friend_id
left outer join cte_friends as f3 on f3.user_id = f2.friend_id and f3.friend_id = f1.user_id
where
f1.user_id = 1 and f3.user_id is null and
f2.friend_id != 1
group by f2.friend_id
order by 2 desc