What is the average number of students for each advisor?
students refers to advisedBy.p_id; advisor refers to p_id_dummy; average number = avg(count(advisedBy.p_id))
SELECT CAST(COUNT(p_id) AS REAL) / COUNT(DISTINCT p_id_dummy) AS avgnum FROM advisedBy GROUP BY p_id_dummy
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, 
