Easy solution using Union and where clause in MYSQL
\nselect  product_id, \'store1\' as store, store1 as price from Products  where store1 is not null\nunion\nselect  product_id, \'store2\' as store, store2 as pr
```\nselect  product_id, \'store1\' as store, store1 as price from Products  where store1 is not null\nunion\nselect  product_id, \'store2\' as store, store2 as price from Products  where store2 is not null\nunion\nselect  product_id, \'store3\' as store, store3 as price from Products  where store3 is not null\n```
['Union Find', 'MySQL']