sql - How to implement total disjoint specialization in database?

Question

Say there is a database for students and professors (a very simple one) and the relational database is the following:

GradStudent (_id_, name, gradStuff)
UndergradStudent (_id_, name, underGradStuff)
Professor (_id_, name)
Teaches(_prof_id_, _stud_id_)

Considering that the relational database above is meant to represent total disjoint specialization, i.e. there is no table Student but rather two completely seperate ones, when writing this in SQL in order to implement the database, how would I fetch the student id for the Teaches table? I cannot figure out how to make a foreign key from two different tables.

I am writing this question assuming that the SQL language is not all the different across all the platforms. If clarification is needed: I am working on Oracle SQL Developer.

Edit :: additional info / Clarification:

For a more graphical, simplistic view on what I am trying to achieve:

I want to write the following in SQL code (however I do not know how is it possible and thus don't know how to)

My apologies if the picture is too simplistic, if needed I can add more attributes and details, just let me know.

Answer 1

If by "fetch the student id for the Teaches table", you mean you want Teaches.stud_id to be a FK that references "GradStudent or Undergradstudent as is the case", you can't. The target of a FK must be a key of a table that is not a view. You have no such table, ergo you have no such key either.

Only way I see is to code a trigger that does the check upon inserts/updates to Teaches.