MySQL union
\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 pric
```\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```
['MySQL']