Re: On view updating
Date: Mon, 20 Sep 2004 21:45:02 +0100
Message-ID: <414f414e$0$69726$ed2619ec_at_ptn-nntp-reader01.plus.net>
Costin Cozianu wrote:
>> Less powerful, you say? Can you explain this? I spent the weekend reading >> papers describing the relational algebra as being exactly as powerful as >> the relational calculus which is in turn exactly as powerful as the >> first order predicate logic. But you are saying "less powerful." Can you >> help me resolve this paradox?
>
> Sure very simple. Try to let the user input the known fact that
> X=2 OR X=3. Say "mary has blue eyes or mary has green eyes".
>
> Or to say that "Whoever has blue eyes will be likable to john"
A user at that level though can't input *any* facts at all without the help of someone else with the ability to use DDL - to define the tables in the first place.
So what it means is that DML users are restricted to being able to enter a subset of logical facts into the database. Are there any statements of first order logic that can't be entered into a database if you include DDL?
I guess constraints fall into a grey area between data and metadata (at least for standard SQL databases). You can't really query constraints, unless you use the system tables. I've heard of "constraint databases" but never seen a concrete example - could a SQL database be extended to become a constraint database for example, or is it totally different?
Paul. Received on Mon Sep 20 2004 - 22:45:02 CEST