Which professor teaches the highest number of professional or master/graduate courses?
professor refers to taughtBy.p_id; highest number of professional or master/graduate courses refers to max(count(taughtBy.course_id)) where courseLevel = 'Level_500'
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.course_id) DESC LIMIT 1