Tell me the course description of the course named art history.
select course_description from courses where course_name = "Art history"
Tell me the number of students enrolling this course.
select t1.course_name, count(*) from courses as t1 join student_course_enrolment as t2 on t1.course_id = t2.course_id group by t1.course_name where t1.course_name = "Art history"
How many students did not have any course enrollment?
select count(*) from students where student_id not in (select student_id from student_course_enrolment)