Simple SQL Solution Using Union
\nSELECT product_id, \'store1\' AS store, store1 AS price \n    FROM Products \n    WHERE store1 IS NOT NULL\nUNION\nSELECT product_id, \'store2\' AS store, sto
```\nSELECT product_id, \'store1\' AS store, store1 AS price \n    FROM Products \n    WHERE store1 IS NOT NULL\nUNION\nSELECT product_id, \'store2\' AS store, store2 AS price \n    FROM Products \n    WHERE store2 IS NOT NULL\n    UNION\nSELECT product_id, \'store3\' AS store, store3 AS price \n    FROM Products \n    WHERE store3 IS NOT NULL\n```
['MySQL', 'MS SQL Server']