Simple and Easy " SQL UNION" solution
\nselect product_id, "store1" as store , store1 as price from products \nwhere store1 is not null union\nselect product_id, "store2" as store ,store2 as price f
```\nselect product_id, "store1" as store , store1 as price from products \nwhere store1 is not null union\nselect product_id, "store2" as store ,store2 as price from products \nwhere store2 is not null union\nselect product_id, "store3" as store ,store3 as price from products \nwhere store3 is not null\norder by product_id;\n```
['Union Find', 'MySQL']