[MySQL] Simple solution using union all
\nSELECT product_id, \'store1\' AS store, store1 AS price FROM Products WHERE store1 IS NOT NULL\nUNION ALL \nSELECT product_id, \'store2\' AS store, store2 AS 
```\nSELECT product_id, \'store1\' AS store, store1 AS price FROM Products WHERE store1 IS NOT NULL\nUNION ALL \nSELECT product_id, \'store2\' AS store, store2 AS price FROM Products WHERE store2 IS NOT NULL\nUNION ALL\nSELECT product_id, \'store3\' AS store, store3 AS price FROM Products WHERE store3 IS NOT NULL\n```
['Union Find', 'MySQL']