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 A and B, differing by entry. What’s the best way to implement this?

Ideas so far:

• Create the table with two nullable foreign key fields connecting to the two tables.
  • Possibly with a trigger to reject inserts and updates that would result 0 or 2 of them being null.
• Two separate tables with identical data
  • This breaks the rule about duplicating data.

What’s a more elegant way of solving this problem?

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.