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Why does the outer query return nothing if second subquery returns no results?

August 18, 2022

I’m trying to figure out why the outer query returns nothing if the second sub query has no return value even if the first does?

SELECT * FROM
(
    SELECT category_id
    FROM story_category
    WHERE story_category.story_id = 9998
) AS c,
(
    SELECT typology_id
    FROM story_typology
    WHERE story_typology.story_id = 9998
) AS t;

If I pick another story_id for the second query which returns a typology_id only then does the main query return a results for both subquery:

>Solution :

Having two elements in the FROM list means a Cartesian product or cross join, that is, each row from the left side is combined with each row from the right side. If the left side has n rows and the right side has m rows, the result will have n*m rows.

Consequently, if one of the sets is empty, the result is also empty.