Question

For each student A who likes a student B where the two are not friends, find if they have a friend C in common (who can introduce them!)

I’ve tables

 Friend (ID1, ID2) 

 Likes(ID1, ID2)

I need to select pairs of id from Likes that don’t friends.

I’ve done it

    SELECT L.id1 id1, L.id2 id2
 FROM likes L
 LEFT JOIN friend F ON (L.id1 = F.id1 and L.id2 = F.id2)
 WHERE F.id1 IS NULL

How to find Friend IDs in common?

Answer 1

We need to find all tuples such that the following conditions apply:

1. Person A likes person B
2. Person A is friends with person C
3. Person B is friends with person C
4. Person A is not friends with B, and B is not friends with A

A query that accomplishes this would be something like this:

SELECT L.ID1 as Liker, L.ID2 as Liked, F1.ID2 as MutualFriend
FROM Likes L, Friend F1, Friend F2 
WHERE F1.ID2 = F2.ID2
AND F1.ID1 = L.ID1
AND F2.ID1 = L.ID2
AND NOT EXISTS(
    SELECT * FROM FRIEND F3
    WHERE (F3.ID1 = L.ID1 AND F3.ID2 = L.ID2)
    OR (F3.ID1 = L.ID2 AND F3.ID2 = L.ID1));

PS I only tested in in sqlite3, so there may be a slight syntax error in another DBMS, but I don’t think so. Also, my answer returns a slightly more readable answer than just the mutual friend id.

Hope that helps,