Nancy,

RI is a data issue, the conditions and approaches to enforcing it are business in nature. Referential Integrity simply refers to protecting the integrity of the PK to FK references within the database. RI itself doesn't say anything about how to protect the references. Most databases will give you simple abilities for declaring RI rules, the standard cascade or restrict. However, most business situations are not that simple. There are situation where a rule would be cascade and others where it would be restrict and still others where it might be ignore.

Part of this discussion is rooted in the idea of 3-tier architecture. Once one beraks from the 3-tier thinking the options expand considerably. If one considers a five tier architecture;

User Interface Layer

Input/Output Layer

Business Layer

Data Services (Access) Layer

Database Layer

Then it becomes more clear that the optimal location for RI rules is in the Data Services or Data Access layer.