Which student registered for the least number of courses? Give the student id.
select student_id from student_course_registrations group by student_id order by count(*) asc limit 1
Most number of courses?
select student_id from student_course_registrations group by student_id order by count(*) desc limit 1
Give the student details.
select t1.student_details from students as t1 join student_course_registrations as t2 on t1.student_id = t2.student_id group by t1.student_id order by count(*) desc limit 1