Combine Records from Different Tables

Let's say you have 2 users and 3 categories and you want a query to return the combination of all the records, resulting in 6 rows.

You can use cross join to do that:

select
  users.id as user_id,
  categories.id as category_id
from users cross join categories


 user_id | category_id
---------+-------------
       1 |           1
       1 |           2
       1 |           3
       2 |           1
       2 |           2
       2 |           3