compound propositions
Date: Mon, 15 Mar 2010 19:12:55 GMT
Message-ID: <XEvnn.71100$PH1.31420_at_edtnps82>
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?
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.
(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 - 20:12:55 CET