fn main()

select SUM(CASE WHEN (DATE_FORMAT(order_date,'%m-%d-%y') > DATE_FORMAT(CURRENT_DATE - INTERVAL 30 DAY,'%m-%d-%y')...

Are you want to program some algorithms or some basic CS concepts ? I would say you can go and try it in https://www.hackerrank.com

Its very interesting to watch GeeksofGeeks videos, if you are interested to learn algorithms and some cool solution for problems.