Re: Codd's Information Principle

On Nov 5, 10:54 am, Cimode <cim..._at_hotmail.com> wrote:
> I would be glad to hear how we establish a valid quantifier in
> relational algebra using only internal predicates. The lack of
> clarification of the external predicate, while being symptomatic
> limitation of traditional RM relational theorists gladly recognize,
> does not bother them much when it comes to operate relations
> algebrically using only the internal predicate.

Please read Bob's recent postings and especially my posting of Oct 28 http://groups.google.ca/group/comp.databases.theory/browse_thread/thread/ebd880c6b29cddbd/1cc4b80087f296a2?hl=en#1cc4b80087f296a2

A query evaluates the extension of (ie tuples that satisfy) a predicate expression (the one corresponding to the query relation expression) built from external (ie base relation variable) predicates.
An internal predicate is just a necessary but not necessarily sufficient constraint on the tuples that can appear in a variable (evaluated to avoid (some) erroneous inputs). It has nothing to do with querying.
(I don't even find the notion of internal predicates helpful. It's the overall database constraint that's important.)

On Nov 5, 3:08 pm, Cimode <cim..._at_hotmail.com> wrote:
> The concept that a relational *operation* (projection) involving a
> relation R1 would also serve as a quantifier for the same relation is
> a concept I am having difficulties with.

Use of a relation operation in a relation expression corresponds to use of a connective or quantifier in the corresponding predicate expression.
Read my referenced message.
Try an example.
It's all so straightforward.

philip Received on Fri Nov 06 2009 - 01:19:53 CET