If they stumble on the question, circle back to the Orders and OrderDetails tables we used as examples earlier. What’s an orphan? How do we make sure that we don’t end up with OrderDetails for records with no matching Order record? Where are all the places we could enforce referential integrity? (Think foreign keys, triggers, the application, or not at all.) Have you worked in places where there was no referential integrity, and what problems did you run into?