Mysql double select or inner join - mysql

Question

I have two MySql tables.
One of them stores general data about a product (name, image, description).
table name: product
table fields: id, name, desc, brand, image, status
The second one stores different variations about products (size, color, price, promotion price).
table name: product_variation
table fields: product_id, size, color, price, prom_price, status
The thing I don't realy know is how to display the products from the first table ordered by the prices from the second table, considering that in the second table there are more than one rows with the same product id, but different sizes, color and of course prices. By default the price field should be considered, but if there is a data larger then 0 in the prom_price field, than prom_price should be considered.
EDIT: the result I need is one instance of every product from the first table, with the lowest possible price from the second table (from all variations of that product in the second table)

Answer 1

SELECT * FROM product p
INNER JOIN product_variation pv
ON p.id = pv.product_id
ORDER BY pv.prom_price, pv.price
This will duplicate your product row for however many variants you have (and show all of these variants in the related fields as well). It orders by prom_price at first and if it's 0 (or not unique) then orders by price.
Edit: You might want to GROUP BY p.id if you want the products kept together.

Answer 2

This is one way of doing it, find the cheapest price on a product (price or prom_price) using a left join, and order by that price. The repeating CASE expressions would be easy to remove using common table expressions, but sadly, MySQL does not have those :-/
SELECT p.name FROM products p
JOIN product_variations pv1 ON p.id=pv1.product_id
LEFT JOIN product_variations pv2 ON pv1.product_id = pv2.product_id
AND CASE WHEN pv1.prom_price<>0 THEN pv1.prom_price ELSE pv1.price END >
CASE WHEN pv2.prom_price<>0 THEN pv2.prom_price ELSE pv2.price END
WHERE pv2.product_id IS NULL
ORDER BY CASE WHEN pv1.prom_price<>0 THEN pv1.prom_price ELSE pv1.price END;
An SQLfiddle to test with.