Neat and Easy Solution
#Solution\n\nselect * from (\nselect product_id,\'store1\' as store, store1 as price from Products\nunion \nselect product_id,\'store2\' as store, store2 as pri
**#Solution**\n\nselect * from (\nselect product_id,\'store1\' as store, store1 as price from Products\nunion \nselect product_id,\'store2\' as store, store2 as price from Products\nunion\nselect product_id,\'store3\' as store, store3 as price from Products\n) store where price is not null;
['Union Find', 'MySQL']