CREATE TABLE student_course_registrations (course_Id VARCHAR)
CREATE TABLE courses (course_name VARCHAR, course_id VARCHAR)

which course has most number of registered students?
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
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 NULLS LAST LIMIT 1