MySQL Easy Level
\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\n```
['Union Find', 'MySQL']