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
database name:db_id computer_student
course table course Cols: course id dtype integer, course level dtype text, 
person table person Cols: person ID dtype integer, professor dtype integer, student dtype integer, has position dtype text, in phase dtype text, years in program dtype text, 
advisedBy table advisedBy Cols: person id dtype integer, person id dummy dtype integer, 
taughtBy table taughtBy Cols: course ID dtype integer, person ID dtype integer, 
