Easy and Simple Solution Using Union
\nselect product_id,\'store1\' store , store1 price from products where store1 is not null\nunion\nselect product_id,\'store2\' store , store2 price from produc
```\nselect product_id,\'store1\' store , store1 price from products where store1 is not null\nunion\nselect product_id,\'store2\' store , store2 price from products where store2 is not null\nunion\nselect product_id,\'store3\' store , store3 price from products where store3 is not null \norder by product_id, price\n\n\n```
['Union Find', 'MySQL']