What are the course names?
select course_name from courses
Which course has the least number of registered students?
select t1.course_name from courses as t1 join student_course_registrations as t2 on t1.course_id = t2.course_id group by t1.course_id order by count(*) asc limit 1
Most number?
select t1.course_name from courses as t1 join student_course_registrations as t2 on t1.course_id = t2.course_id group by t1.course_id order by count(*) desc limit 1