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@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.

(I don't know if it matters that if his approach is right, projection wouldn't depend on whatever the name of the single attribute is - I presume that it would have the same name and type as all headers in the db, eg., just another axiom in the form of the catalog.)

p Received on Thu Aug 17 2006 - 18:52:56 CDT