SQL Simple and Easy solution using Union || Easy Understandinng
Select product_id, "store1" as store, store1 as price\nFrom Products where store1 is not null\nUnion\nSelect product_id, "store2" as store, store2 as price\nFro
Select product_id, "store1" as store, store1 as price\nFrom Products where store1 is not null\nUnion\nSelect product_id, "store2" as store, store2 as price\nFrom Products where store2 is not null\nunion \nSelect product_id, "store3" as store, store3 as price\nFrom Products Where store3 is not null;
['Union Find', 'MySQL']