MySQL solution
\nselect product_id, "store1" store, store1 as price from products where store1 is not null\nunion\nselect product_id, "store2" store, store2 as price from prod
```\nselect product_id, "store1" store, store1 as price from products where store1 is not null\nunion\nselect product_id, "store2" store, store2 as price from products where store2 is not null\nunion\nselect product_id, "store3" store, store3 as price from products where store3 is not null\n```
['Union Find', 'MySQL']