MySQL UNION solution faster than 99%
\nSELECT\nProducts.product_id,\n\'store1\' as store,\nstore1 as price\nFROM Products\nWHERE store1 IS NOT NULL\n\nUNION\n\nSELECT\nProducts.product_id,\n\'store
```\nSELECT\nProducts.product_id,\n\'store1\' as store,\nstore1 as price\nFROM Products\nWHERE store1 IS NOT NULL\n\nUNION\n\nSELECT\nProducts.product_id,\n\'store2\' as store,\nstore2 as price\nFROM Products\nWHERE store2 IS NOT NULL\n\nUNION\n\nSELECT\nProducts.product_id,\n\'store3\' as store,\nstore3 as price\nFROM Products\nWHERE store3 IS NOT NULL\n\n```
['Union Find', 'MySQL']