Re: compound propositions
Date: Tue, 16 Mar 2010 13:15:10 -0700 (PDT)
On Mar 16, 7:12 pm, paul c <toledobythe..._at_oohay.ac> wrote:
> Must admit I've never understood what is the important difference
> between a tuple and an entity, other than that some people would rather
> talk about entities instead of tuples.
My current understanding is that a tuple is a proposition which may describe one or more entities, while an entity is an abstraction which may be described by any number of tuples in any number of relations.
> So in somebody else's vernacular
> maybe I am assuming an 'entity point of view', but that choice of term
> doesn't seem significant to me.
When you said "'C1' is a customer OR 'C1' is a client", it seemed to me you were describing roles or properties of an entity or entity type C1. In a relational database, such roles can be described via separate relvars on which we can define referential constraints to prevent the conjunction of the predicates from being asserted for any C1.
> (I'm not even sure that talking about
> tuples matters as far as the user or programming interface is concerned,
> eg., when talking only about the logical theory it matters but
> probably not at other times.)
> If those relations were 'exclusive', I think the proposition would be
> something like ( 'C1' is a customer and 'C1' is not a client ) OR ( 'C1'
> is not a customer and 'C1' is a client ).
I'm sorry, I don't follow. Would you mind rephrasing, please? Received on Tue Mar 16 2010 - 21:15:10 CET