Question: Who are the top 5 professors who teaches the highest number of professional or master/undergraduate courses?
SQL: SELECT T2.p_id FROM course AS T1 INNER JOIN taughtBy AS T2 ON T1.course_id = T2.course_id WHERE T1.courseLevel = 'Level_500' GROUP BY T2.p_id ORDER BY COUNT(T2.p_id) DESC LIMIT 5