Mutual friends sql

Question

I've seen multiple SO posts on mutual friends but I've structured my friends table in my db so that there are no duplicates eg (1,2) and not (2,1)

    Create Table Friends(
      user1_id int, 
      user2_id int
    );

and then a constraint to make sure user1 id is always smaller than user2 id eg 4 < 5

Mutual friends sql with join (Mysql)

I see suggestions that to find mutual friends it can be found using a join, so this is what I have but I think it's wrong because if I count the data in my db with the actual result from the query I get different results

select f1.user1_id as user1, f2.user1_id as user2, count(f1.user2_id) as 
mutual_count from Friends f1 JOIN Friends f2 ON 
f1.user2_id = f2.user2_id AND f1.user1_id <> f2.user1_id  GROUP BY
f1.user1_id, f2.user1_id order by mutual_count desc

Answer 1

There are three join scenarios that I can see.

1 -> 2 -> 3    (mutual friend id between other IDs)    
2 -> 3 -> 1    (mutual friend id > other IDs)    
2 -> 1 -> 3    (mutual friend id < other IDs)    

This can be resolved with this predicate...

ON f1.user1_id IN (f2.user1_id, f2.user2_id)
OR f1.user2_id IN (f2.user1_id, f2.user2_id)
AND <not joining the row to Itself>

But that will totally mess up the optimiser's ability to use indexes.

So, I'd union multiple queries.

(pseudo code as I'm on a phone)

SELECT u1, u2, COUNT(*) FROM
(
    SELECT f1.u1, f2.u2 FROM f1 INNER JOIN f2 ON f1.u2 = f2.u1 AND f1.u1 <> f2.u2
    UNION ALL
    SELECT f1.u1, f2.u1 FROM f1 INNER JOIN f2 ON f1.u2 = f2.u2 AND f1.u1 <> f2.u1
    UNION ALL
    SELECT f1.u2, f2.u2 FROM f1 INNER JOIN f2 ON f1.u1 = f2.u1 AND f1.u2 <> f2.u2
) all_combinations
GROUP BY u1, u2

Each individual query will then be able to fully utilise indexes. (Put one index on u1 and another index on u2 )

The result should be less esoteric code (with fairly long CASE statements) and a much lower costed execution plan.