Re: Normalisation

Jon Heggland wrote:
> The quote didn't mention nested relations, it is about sets. Do you make
> that assumption about sets as well? Why? Why not about strings?

From my view: if you have a domain of relations, they exist in a separate universe to the main relations in the DBMS.

From your view: relations in a domain of relations exist in the same universe as the main relations of the DBMS.

The inner structure of sets or strings is hidden from the relational operators, so why make an exception for a relation-valued domain?

If you think about the mapping from predicate logic to the relational model, a "nested" relation in the way you propose corresponds to second-order logic.

First order logic basically means that the arguments to your propositions can't themselves be propositions.

But this is the crucial bit I think: they can be values (strings) that can be interpreted as propositions by some other structure.

For example consider the statement:

"Fred has employee_id 123 and the following set of statements are true: {'Fred has phone number 2345', 'Fred has phone number 3456' }"

from the nested viewpoint, the relational part of the DBMS natively understands the second-order reference to the propositions referred to inside the outer proposition.

from the atomic viewpoint, the information about the phone numbers is just a meaningless domain value to the relational part of the DBMS - it needs the type engine to make sense of it.

Now it is possible I guess that the nested relational model adds that little bit of second order logic in such a way that things are still consistent and you also get increased expressive power, but I've not really seen any example of this. It would entail somehow joining an "nested" relation with a standard relation I think.

Paul. Received on Wed Jul 06 2005 - 21:08:24 CEST