>>Bob, I plan to have the business logic be the primary enforcer of this rule, but I also wanted to enforce it at the database level. So would the best way to "fail" the update be to make the trigger throw an exception?
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>In my opinion - yes, it's better to do on the database level in addition to front-end.

That's an approach I started to take after watching some consultants in action. No form of database-level rules, constraints, or relationship rules = a lot of orphaned records and a freaking mess.