which course has most number of registered students?
create table courses (course_name varchar, course_id varchar, PRIMARY KEY (course_name)); create table student_course_registrations (course_id varchar, PRIMARY KEY (course_id))
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