Your question is Normalization, OLAP vs OLTP. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Explain why you normalize a database, the difference between OLAP and OLTP, and the purpose of CTEs; then explain how you would count duplicates in a column.