MySQL Solution
In this case, it is more efficient to use UNION ALL, since there is no need to check for duplicates and spend extra time on it.\n\nSELECT product_id, \'store1\'
In this case, it is more efficient to use UNION ALL, since there is no need to check for duplicates and spend extra time on it.\n```\nSELECT product_id, \'store1\' AS store, store1 AS price\n    FROM Products\n    WHERE store1 IS NOT NULL\n\nUNION ALL\n\nSELECT product_id, \'store2\' AS store, store2 AS price\n    FROM Products\n    WHERE store2 IS NOT NULL\n\nUNION ALL\n\nSELECT product_id, \'store3\' AS store, store3 AS price\n    FROM Products\n    WHERE store3 IS NOT NULL\n```
['MySQL']