Hello! How many students are there in total?
select count ( * ) from students
Great!  Can you list the ids of these students, and filter the list by only the students who are registered for one or more courses?
select distinct t1.student_id from students as t1 join student_course_registrations as t2 on t1.student_id = t2.student_id
Awesome!  Can you also add to this list the number of registered courses for each of these students?
select t1.student_id, count ( * ) from students as t1 join student_course_registrations as t2 on t1.student_id = t2.student_id group by t1.student_id