Question

Buy has a foreign key product_id.

So in addition to SELECT * FROM products; we are interested in getting the number of buys each product has.

Answer 1

Join

SELECT Products.*, COUNT(Buy.ID) AS Bought
FROM products
    LEFT OUTER JOIN Buy ON products.ID = Buy.product_id
GROUP BY Buy.product_id

Subquery

SELECT 
    Products.*, 
    (SELECT COUNT(ID) FROM Buy WHERE product_id = Products.ID) AS Bought
FROM products