advisor(s_id, i_id) connects students and instructors. takes(id, course_id, …) connects students and courses.

1. Which instructors advise more than one student? Return i_id, then the number of students.
2. At most how many advisors can one student have? The key icon tells you.
3. What is the cardinality of advisor, from student to instructor?
4. Which students have no advisor? Return their id.
5. Is the participation of student in advisor total or partial?
6. What is the cardinality of takes between student and course?