As in the last lab, student S209 has an advisor row with a NULL i_id.

1. The names of the students who have at least one course in progress (NULL grade).
2. The names of the instructors who advise nobody. Use NOT EXISTS.
3. The titles of the courses that have at least one prerequisite (see the prereq tab).
4. The names of the students who take a course of a department other than their own department.