Easy Approach || Suggest Better Meathods
\tselect product_id, \'store1\' as store, store1 as price from products where store1 is not null\n\tunion\n\tselect product_id, \'store2\' as store, store2 as p
\tselect product_id, \'store1\' as store, store1 as price from products where store1 is not null\n\tunion\n\tselect product_id, \'store2\' as store, store2 as price from products where store2 is not null\n\tunion\n\tselect product_id, \'store3\' as store, store3 as price from products where store3 is not null
['MySQL']