What is the average number of courses taught by a professor?
professor refers to professor = 1; average number of courses = divide(count(taughtBy.course_id), count(taughtBy.p_id) where professor = 1 )
SELECT CAST(COUNT(T1.course_id) AS REAL) / COUNT(DISTINCT T2.p_id) AS num FROM taughtBy AS T1 INNER JOIN person AS T2 ON T1.p_id = T2.p_id WHERE T2.professor = 1