📘 Intermediate — Aggregations, Joins, UDFs & Caching
1. How do you group by department and count employees?
2. Which function allows custom column logic?
3. How do you cache a DataFrame?
4. Which join returns only matching rows?
5. Apply UDF convert_salary to salary column?
6. Persist DataFrame to memory and disk?
7. Remove duplicates based on columns?
8. Rename a DataFrame column?
9. Difference between map and flatMap?
10. How to perform a left join on id?