Simple to understand | 40%  faster | easy code
```\n# Write your MySQL query statement below\nselect product_id, \'store1\' as store ,store1 as price \nfrom Products\nwhere store1 is not null\nunion \nselect
```\n# Write your MySQL query statement below\nselect product_id, \'store1\' as store ,store1 as price \nfrom Products\nwhere store1 is not null\nunion \nselect product_id, \'store2\' as store ,store2 as price \nfrom Products\nwhere store2 is not null\nunion\nselect product_id, \'store3\' as store ,store3 as price \nfrom Products\nwhere store3 is not null;\n
['Union Find', 'MySQL']