Join query with IN condition and Group_concat
I'm not 100% sure if I understand the question correct, but i'll try some solution!:

    SELECT a.product_id, group_concat(distinct b.name)
    FROM product_to_category a
    JOIN category_description b
    ON a.category_id = b.category_id
    GROUP BY a.product_id;


I hope this is what you were looking for...

Cheers,
Olaf