James,

>Is a Candidate Key a form of Business Rule? Are Referential Integrity rules a form of Business Rules?

That's a good question. Personally I think the border between database rules and business rules are rather vague. I do regard RI rules and uniqueness constraints (PK and C(andidate)K(eys) as a part of the database design). However I'm fully aware that the RI implementation (Restrict, Cascading, Nulliefies or ignore) depends on the businessrules.

This is a good topic for discussion.

>At the moment, I am using VFP to design the user services, business services and data services layers of my applications. I want to make sure that my design allows me the greatest possible independence of databases and programming languages. I would like to be able to move the data to Microsoft SQL Server, Oracle or almost any other back-end database. I would like the ability to design a front end using Visual Basic, ASP or almost any other development system.

>I would appreciate advice on whether to enforce the uniqueness of key fields or expressions in a 3 (or more) tier structure in the data services or the business services layer. Similarly, should Referential Integrity be maintained in the data services or business services layer?

Since all popular (R)DBMS support data services implemented on the back-end as Stored procedures, I would implement this in the data services layer. Since the back-end will support RI and uniqueness constraint far better as you can do within VFP I'll say this is the most efficient choice.

Walter,