>>Agreed. But what is a business rule vs. a RI rule. One client may say that he needs to have a customer for every invoice that is entered and another may say that they need to be able to enter an invoice and associate it with a client later(idea from 'About Face').
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>Ned,
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>For me database rules are inherent in the databse design while business rules vary according to the business. In the case of invoices and customers I would enforce RI as a 0-1 for invice to customer and then use the business layer to enforce those clients that want 1-1. That way the data design works for all situations while the business rule can change from client ot client.

Makes sense. One question, what do you mean by a 0-1 relationship, and how is it enforced?
TIA,
Ned