Re: compound propositions

paul c wrote:

> Some months ago Bob B took me to task for language that might have been
> too loose or even glib, referring to predicates and expressions. That
> was fair enough. Though I could've made it clearer that by expressions
> I meant relational algebra expressions, I still don't have clear answers
> to all of his complaint.
>
> One reason is that I still don't know how Codd's Information Principle
> applies to compound propositions, eg., " 'C1' is a customer OR 'C1' is a
> client". I can see that humans might imagine themselves capable of
> interpreting a relation (or to put it redundantly a relation value) as
> implitly mentioning that 'OR' connective (and dba's might so instruct
> their users). But where is it recorded? (or 'manifested'?) Eg., is it
> 'recorded' only in the ephemeral form of an expectation that a program's
> execution can't manifest given a single relation to operate on?

Where does one record any external predicate?

> As far as I can tell, there is no way to record a logical connective in
> a tuple, therefore not for a tuple and therefore not in a relational
> value (other than in a disconnected text mode thath isn't amenable to
> the algebra) which has always made me suspect that Codd's R-tables don't
> store compound propositions. If so, that would be one difference
> between internal and external predicates, which would make me suspect
> that we can't always expect the same results when the same algebra is
> applied to both.

What is the predicate of Customer[id] join Client[id] where [] signifies project? If the expression defined a view, what might you call it?

> (I realize that Codd and others - maybe Ullman, I forget - showed that
> FOL and his relational algebra were equivalent, but I presume the
> conditions of that were with reference to his R-tables and not that FOL
> always gives the same results under all conditions.)
Received on Mon Mar 15 2010 - 20:39:37 CET