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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 as user_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
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