MySQL: union + subquery (the simplest solution)
\nselect * from \n(select product_id, "store1" as store, store1 as price from Products\nunion \nselect product_id, "store2", store2 from Products\nunion\nselect
```\nselect * from \n(select product_id, "store1" as store, store1 as price from Products\nunion \nselect product_id, "store2", store2 from Products\nunion\nselect product_id, "store3", store3 from Products) sub where price is not null;\n\n```
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