Q:

Representing an Entity Type with two parent types in an Entity-Relationship Diagram?

I have entity type called STUDENT which has two sub-types: GRADUATE, and UNDERGRADUATE. Easy enough.
Now, I (presumably based on specifications) also have another entity type called RESEARCH ASSISTANT, which is defined as being a student (thus inheriting STUDENT's attributes)... but the specification is ambiguous as to whether RESEARCH ASSISTANT is either a graduate or an undergraduate.
Assuming that a research assistant can be either case, how do I represent this in an entity relationship diagram? In examples, the notation represents a child type by drawing it within the parent... But what if the child has two different parents?
If I simply sub-type STUDENT, then it becomes ambiguous as to whether the research assistant is graduated or not... So do I draw two different entities for RESEARCH ASSISTANT (one within graduate and the other within undergraduate)? :S

A:

You can think it in another way. Instead of making a research assistant inherit from student, you can make it into a relationship with those (what in OOP is known as composition over inheritance)
In the diagram, keep student, graduate and undergraduate just as you have right now, and add a "one to zero or one" relationship from student to research assistant. When that relation is present, the student (graduate, undergraduate or whatever) will also be an assistant, but when that relation is missing he'll be student only.

