CREATE TABLE students (student_id VARCHAR2)
CREATE TABLE student_course_registrations (student_id VARCHAR2)

List the id of students who registered some courses and the number of their registered courses?
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
SELECT "T1"."student_id", COUNT(*) FROM "students" "T1" JOIN "student_course_registrations" "T2" ON "T1"."student_id" = "T2"."student_id" GROUP BY "T1"."student_id"