Easy To understand
Intuition\n Describe your first thoughts on how to solve this problem. \n\n# Approach\n Describe your approach to solving the problem. \n\n# Complexity\n- Time 
# Intuition\n<!-- Describe your first thoughts on how to solve this problem. -->\n\n# Approach\n<!-- Describe your approach to solving the problem. -->\n\n# Complexity\n- Time complexity:\n<!-- Add your time complexity here, e.g. $$O(n)$$ -->\n\n- Space complexity:\n<!-- Add your space complexity here, e.g. $$O(n)$$ -->\n\n# Code\n```\n\nselect product_id,\'store1\' as store,store1 as price\nfrom products where store1 is not null\nunion all\nselect product_id,\'store2\' as store,store2 as price\nfrom products where store2 is not null\nunion all\nselect product_id,\'store3\' as store,store3 as price\nfrom products where store3 is not null\n\n\n\n```
['PostgreSQL']