Re: compound propositions
Date: Wed, 24 Mar 2010 09:17:39 -0400
"paul c" <toledobythesea_at_oohay.ac> wrote in message
> 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?
I'm confused. Are you trying to find a way to avoid using nulls? When you record a disjunction, information is inherently missing: it is known that at least one disjunct is supposed to be true, but not which.
I think that if you recast what you intend in terms of nulls, and then seek to eliminate those nulls, you'll find that the resulting scheme doesn't involve the recording of disjunctive information.
On the other hand, you could find what it is that is in common between the disjuncts. Both customers and clients are payors, for example.
> 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
> (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 Wed Mar 24 2010 - 14:17:39 CET