extracting information from schemas

Dear all,

    If there are 3 relations as shown below: PreferredTutor(name varchar(40), instructor varchar(40)) PreferredInstructor(name varchar(40), tutor varchar(40)) Assignment(courseTitle varchar(40), instructor varchar(40))

A tuple(x,y) is in PreferredTutor if an instructor x prefers a tutor y to work on his course.
A tuple(x,y) is in PreferredInstructor if a tutor x prefers to work for the instructor y.

In reality, tutors may not have any preferred instructor to work with. tutors who do not indicate his preferred instructor and is not preferred by any instructor will not have his or her name listed in any of the above 3 tables. Suppose new tables are not allowed to be built, how can I ensure that all tutor names appear somewhere in the tables?

And how can I find the total number of tutors using only the above 3 tables?

    Thx a lot! Received on Sat Dec 02 2000 - 12:54:12 CST