sql query to collect users having common items

Question

I'm facing a problem with Postgres. Here is the example: i got 3 tables: users, items and boxes

boxes table:

user_id    |   item_id
   1       |      3
   1       |      4
   1       |      6
   1       |      7
   2       |      5
   2       |      10
   2       |      11
   3       |      5
   3       |      6
   3       |      7

Given this boxes table, i would like to retrieve items among users who share minimum 2. So the SQL query result expected should be item_id: 6, 7 because user 1 and user 3 share items 6 and 7. But user 2 and 3 share only one item: the item 5 so item 5 is not in result.

I'm trying so many ways without success. I wonder if someone can help me.

Answer 1

Try this. It returns 6 and 7 (and 5,6,7 if you add a record "1,5"), but I haven't tested it extensively.

-- The Outer query gets all the item_ids matching the user_ids returned from the subquery    
SELECT DISTINCT c.item_id FROM boxes c -- need DISTINCT because we get 1,3 and 3,1...
    INNER JOIN boxes d ON c.item_id = d.item_id
    INNER JOIN
        --- the subquery gets all the combinations of user ids which have more than one shared item_id
        (SELECT a.user_id as first_user,b.user_id as second_user FROM
        boxes a
        INNER JOIN boxes b ON a.item_id = b.item_id AND a.user_id <> b.user_id -- don't count items where the user_id is the same! Could just make the having clause be > 2 but this way is clearer
        GROUP BY a.user_id,b.user_id
        HAVING count(*) > 1) s
    ON s.first_user = c.user_id AND s.second_user = d.user_id