Re: compound propositions
Date: Mon, 15 Mar 2010 18:38:17 -0300
paul c wrote:
> 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.
Actually, for join it is "'id' is a customer and a client". For union, it would be "'id' is a customer or a client".
>> 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.
The only difference is a view has a name. Otherwise, it is identical to any other derived relation.
Unless, of course, one is looking at named relations in which case the only difference is a view is derived. Otherwise, it is identical to any other named relation. Received on Mon Mar 15 2010 - 22:38:17 CET