SELECT all items in common between two users on three tables

Question

I have three tables

item_to_user (to store the relations between user and item)

| item_to_user_id | user_id |  item_id |
-----------------------------------------

item_tb

| item_id | item_name |
-----------------------

user_tb

| user_id | user_name |
-----------------------

An item can belong to one or more user and viceversa, that's why I'm using the first table.

So, given the user_id = A and user_id = B how can I do a mysql query to select all the items the belong both to user A and user B?

note: I wrote a similar question yesterday but was about two tables not three.

Answer 1

   SELECT i.*
     FROM item_tb AS i
LEFT JOIN item_to_user AS iu
       ON iu.item_id = i.item_id
LEFT JOIN user_tb AS u
       ON iu.user_id = u.user_id
    WHERE u.user_name IN ('A', 'B')
 GROUP BY i.item_id
   HAVING COUNT(i.item_id) > 1

Answer 2

By prequerying common items between A and B (via count(*) = 2) will pre-limit the final list of items possible to get details from. Then joining that to the items table as SECOND table in the query should help performance. Especially if A&B have 50 common items, but your items table consists of 1000's of items.

select straight_join 
       i.item_id, 
       i.item_name
   from
       ( select iu.item_id
               from item_to_user iu
                  join user_tb u
                     on iu.user_id = u.user_id
                    and u.user_name in ( 'A', 'B' )
               group by 1
               having count(*) = 2 ) Matches,
       item_tb i
   where
      Matches.item_id = i.item_id

Answer 3

If a user can't have repeated items then this simple one will work:

select item_id
from item_to_user
where user_id in ('A', 'B')
group by item_id
having count(*) > 1