Question

I have a products table and a categories table. The categories have a parent-child hierarchy with only 2 levels (parent and child). Products can be associated with many categories via table PC.

How can I find all products that have a parent category but no child from that same parent?

Answer 1

Could look like this:

SELECT p.prod_id, p.prod
FROM   products p
JOIN   pc pcp ON pcp.prod_id = p.prod_id
JOIN   category cp ON cp.cat_id = pcp.cat_id
WHERE EXISTS (
    SELECT *
    FROM   category c0
    WHERE  c0.parent_id = cp.cat_id
    )
AND NOT EXISTS (
    SELECT *
    FROM   pc pcc
    JOIN   category cc ON cc.cat_id = pcc.cat_id
                      AND cc.parent_id = cp.cat_id
    WHERE  pcc.prod_id = p.prod_id
    )
GROUP BY p.prod_id, p.prod

“Find all products with a category that has a child (= parent category) where no child of this category is linked to the same product.”
This assumes that the parent-child relation is implemented with a single column in the table category:

parent_id REFERENCES category (cat_id)

If you were a nicer guy, you would have specified all that in the question.