Zkuste dotaz v tomto smyslu:
SELECT
t1.pro_id,
t2.pro_name,
t2.pro_desp,
t1.price,
t1.pro_model,
t4.cat_desp
FROM products t1
INNER JOIN pro_description t2
ON t1.pro_id = t2.pro_id
INNER JOIN pro_to_category t3
ON t1.pro_id = t3.pro_id
INNER JOIN category t4
ON t3.cat_id = t4.cat_id