CREATE TABLE students (student_id VARCHAR(255)); CREATE TABLE student_course_registrations (student_id VARCHAR(255))
List the id of students who registered some courses and the number of their registered courses?
    
SQL: 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