Simple MySQL solution using UNION
\n\nselect product_id, \'store1\' as store , store1 as price\nfrom Products\nwhere store1 is not null\nUNION\nselect product_id, \'store2\' as store , store2 as
\n```\nselect product_id, \'store1\' as store , store1 as price\nfrom Products\nwhere store1 is not null\nUNION\nselect product_id, \'store2\' as store , store2 as price\nfrom Products\nwhere store2 is not null\nUNION\nselect product_id, \'store3\' as store , store3 as price\nfrom Products\nwhere store3 is not null\n```
['MySQL']