Simple MYSQL Solution
\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
```\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```
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