1. Write a SQL query to find the ID and name of each instructor who has given an 'A' grade in any course that they have taught.
2. Write a SQL query to find the ID and name of each instructor who has NEVER given an 'A' grade in any course that they have taught. (Instructors who have never taught a course trivially satisfy this condition and will appear in the results. That's OK, your friends want to avoid the "unknown" as well.)
3. Write a SQL query to find the course_id of any courses which have been retaken by 1 or more students. In other words, a student has taken the same course more than once.
4. Your friends like to sleep in. Write a SQL query to find the ID and title of each course in 'Comp. Sci.' that has had at least one section with afternoon hours (i.e. time_slot.start_hr >= 12). You should eliminate duplicates from the results, if there are any.