How to handle an “OR” relationship in an ERD (table) design?

Question

I\'m designing a small database for a personal project, and one of the tables, call it table C, needs to have a foreign key to one of two tables, call them

Answer 1

You're describing a design called Polymorphic Associations. This often gets people into trouble.

What I usually recommend:

A  -->  D  <--  B
        ^
        |
        C

In this design, you create a common parent table D that both A and B reference. This is analogous to a common supertype in OO design. Now your child table C can reference the super-table and from there you can get to the respective sub-table.

Through constraints and compound keys you can make sure a given row in D can be referenced only by A or B but not both.