simple union
\nselect product_id,\'store1\' as store, store1 as price from products where store1 is not null\nunion\nselect product_id,\'store2\' as store, store2 as price f
```\nselect product_id,\'store1\' as store, store1 as price from products where store1 is not null\nunion\nselect product_id,\'store2\' as store, store2 as price from products where store2 is not null\nunion\nselect product_id,\'store3\' as store, store3 as price from products where store3 is not null\n```\n
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