Two MySQL Solutions
\nSELECT\n    product_id,\n    \'store1\' AS store,\n    store1 AS price\nFROM\n    Products\nWHERE\n    store1 IS NOT NULL\nUNION\n    SELECT\n        product_
```\nSELECT\n    product_id,\n    \'store1\' AS store,\n    store1 AS price\nFROM\n    Products\nWHERE\n    store1 IS NOT NULL\nUNION\n    SELECT\n        product_id,\n        \'store2\' AS store,\n        store2 AS price\n    FROM\n        Products\n    WHERE\n        store2 IS NOT NULL\nUNION\n    SELECT\n        product_id,\n        \'store3\' AS store,\n        store3 AS price\n    FROM\n        Products\n    WHERE\n        store3 IS NOT NULL;\n```\n\n```\nSELECT\n    product_id,\n    store,\n    price\nFROM\n(\n    SELECT product_id, \'store1\' AS store, store1 AS price FROM Products\n    UNION \n    SELECT product_id, \'store2\' AS store, store2 AS price FROM Products\n    UNION\n    SELECT product_id, \'store3\' AS store, store3 AS price FROM Products\n) p\nWHERE\n    price IS NOT NULL;\n```
['Union Find', 'MySQL']