[Old Pedant]

Sunfighter: That won't work if you has some products and/or categories that do *NOT* have any matching product_extra_categories.

He needs a LEFT JOIN in there in that case.

select
    p.product_id
    p.product_name,
    c.category_name,
    e.category_name
from
    products AS p 
    INNER JOIN categories AS C ON p.product_id = c.product_id
    LEFT JOIN product_extra_categories AS e ON p.product_id = e.product_id

;