Simple MySQL Solution
```\nselect product_id, \'store1\' as store, store1 as price \nfrom products where store1 is not null\nunion\nselect product_id, \'store2\' as store, store2 as 
```\nselect 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;
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