【MSSQL】Easy solution with unpivot
```sql\n-- using unpivot\nselect product_id, store, price\nfrom(\n    select product_id, store1, store2, store3 from Products\n    ) as p\nunpivot(\n          p
```sql\n-- using unpivot\nselect product_id, store, price\nfrom(\n    select product_id, store1, store2, store3 from Products\n    ) as p\nunpivot(\n          price for store in (store1,store2,store3)\n    ) as pv
['MS SQL Server']