Re: Notions of Type
From: paul c <toledobythesea_at_oohay.ac>
Date: Thu, 17 Aug 2006 23:52:56 GMT
Message-ID: <sj7Fg.426224$IK3.379801_at_pd7tw1no>
...
>
> Wait a minute...
>
> PROJECT <relation> <set of attributes> -> <relation>
>
> right? And the question is, "Does this definition of
> PROJECT demonstrate lack of algebraic closure?" I think
>
> PROJECT <relation> <relation> -> <relation>
>
> is a step in the wrong direction and don't think such a
> thing is necessary for closure. I think that the fact
> that one of the operands and the result are relations is
> what provides closure.
Date: Thu, 17 Aug 2006 23:52:56 GMT
Message-ID: <sj7Fg.426224$IK3.379801_at_pd7tw1no>
J M Davitt wrote:
> David Cressey wrote:
>> "Marshall" <marshall.spight_at_gmail.com> wrote in message >> news:1155833602.403082.5690_at_h48g2000cwc.googlegroups.com... >>
...
>> I'm surprised the PROJECT is such a problem. Maybe I should stay out >> of the >> discussion, because this is a little over my head. But here goes, >> anyway: >> >> Why can't you define a "set of attributes" as a relation? I'm thinking >> that an empty relation (one with no tuples) has exactly the same >> information content as a "set of attributes". If you do that, why can't >> you say, >> >> PROJECT <relation>, <empty relation> -> <relation> >> >> >> Or have I violated some other aspect of the formalism?
>
> Wait a minute...
>
> PROJECT <relation> <set of attributes> -> <relation>
>
> right? And the question is, "Does this definition of
> PROJECT demonstrate lack of algebraic closure?" I think
>
> PROJECT <relation> <relation> -> <relation>
>
> is a step in the wrong direction and don't think such a
> thing is necessary for closure. I think that the fact
> that one of the operands and the result are relations is
> what provides closure.
Earlier, Bob B said about the "set of attributes": "It is a relation of degree 1 and cardinality N representing a set of N attribute names". Maybe you are right and saying that isn't necessary, but his stance seems to me to obviate the question.
p Received on Fri Aug 18 2006 - 01:52:56 CEST