Re: compound propositions

From: paul c <>
Date: Mon, 15 Mar 2010 21:28:49 GMT
Message-ID: <lExnn.68191$Db2.30398_at_edtnps83>

Bob Badour wrote:

> What is the predicate of Customer[id] join Client[id] where [] signifies
> project? ...

If you mean what is the 'external' predicate, I can't say, not knowing its purpose any more than a dbms designer can predict the exact purpose some unknown db is used for, other than to say intersection and quantification are involved. If you mean 'internal', that which an algebra operates with, it is basically the expression 'Customer[id] join Client[id]', standing for the intersection of the set of Customer id's that match Client id's and vice-versa.

If the expression defined a view, what might you call it?
> ...

You might call it a predicate that's satisfied by certain compound propositions. I think you might call it that even if it is 'satisfied' by tuples of a 'base' 'relvar'. I don't see that an expression being a view definition makes it stand for some different kind of predicate, if that's what you're driving at. I didn't think that views needed to be mentioned but since they were, views seem a programming convenience, eg., a labour-saving device or a way to effect some security attitude or a way to control physical storage, or a way to help an optimizer or even all four at once, maybe other conveniences too that I can't think of right now. Are they something more than that? Eg., do they have special significance in relational algebra? Received on Mon Mar 15 2010 - 22:28:49 CET

Original text of this message