Unpivot columns into rows

Table products:

Turn the three price columns into rows: one row per product and store where the price is not NULL. A price of 0 is a real price and stays.

Output columns: product_id, store ('store1', 'store2' or 'store3'), price.

CREATE TABLE products (
  id int PRIMARY KEY,
  store1 int,
  store2 int,
  store3 int
);