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 AS T1 JOIN student_course_registrations AS T2 ON T1.student_id = T2.student_id GROUP BY T1.student_id
[{'student_id': 111, 'count(*)': 1}, {'student_id': 121, 'count(*)': 2}, {'student_id': 131, 'count(*)': 1}, {'student_id': 141, 'count(*)': 2}, {'student_id': 151, 'count(*)': 1}, {'student_id': 161, 'count(*)': 1}, {'student_id': 171, 'count(*)': 1}]