Question

This seems interesting.

Assume three tables, users, items and a join table users_items. The latter links users.ids to items.ids. We say that user U owns item I when there’s a row in the users_items table that contains the pair (U.id, I.id).

Now: how would I go about randomly choosing an item that a particular user does not yet own?

The most feasible solution seems to be:

1. gather all item ids.
2. gather ids of items owned by user U.
3. remove all ids from step #2 from the set generated in #1
4. pick random element from the resulting set (trivial)

Is this the only sane way? Is it optimal? Let’s assume this would be a heavily used feature. And the numbers of elements in each table are huge.

Any fun ideas, thoughts?

Answer 1

You can use a left join to generate a list of item ids that the user does not currently own by doing :

SELECT i.id
FROM items i 
LEFT JOIN users_items ui ON ( i.id = ui.item_id AND ui.users_id = 2 )
WHERE ui.item_id IS NULL;

From here, you can randomly select one of the elements from the result set.