How many students are there?
select count ( student_id ) from student_course_registrations
what about courses?
select count ( course_id ) from student_course_registrations
how many students registered for just one course
select count ( * ) from ( select student_id from student_course_registrations group by student_id having count ( * ) = 1 )
what are their ids?
select student_id from student_course_registrations group by student_id having count ( * ) = 1