EASY TO UNDERSTAND | MySQL Solution
```\nSELECT product_id, \'store1\' AS store, store1 AS price \nFROM products WHERE store1 IS NOT NULL\nUNION\nSELECT product_id, \'store2\' AS store, store2 AS 
```\nSELECT product_id, \'store1\' AS store, store1 AS price \nFROM products WHERE store1 IS NOT NULL\nUNION\nSELECT product_id, \'store2\' AS store, store2 AS price \nFROM products WHERE store2 IS NOT NULL\nUNION \nSELECT product_id, \'store3\' AS store, store3 AS price \nFROM products WHERE store3 IS NOT NULL
['MySQL']