How many registered students are there?
select count(distinct student_id) from student_course_registrations
How many are there for each course?
select count(*) from student_course_registrations group by course_id
Give the course name as well.
select t3.course_name, count(*) from students as t1 join student_course_registrations as t2 on t1.student_id = t2.student_id join courses as t3 on t2.course_id = t3.course_id group by t2.course_id