Re: Notions of Type

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Fri, 18 Aug 2006 13:51:46 GMT
Message-ID: <SBjFg.51098$pu3.597337@ursa-nb00s0.nbnet.nb.ca>

David Cressey wrote:

> "paul c" <toledobythesea_at_oohay.ac> wrote in message
> news:sj7Fg.426224$IK3.379801_at_pd7tw1no...
>
>

>>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.)

>
>
> I tend to think like a computer person and not like a mathematician. What
> follows reflects this.
>
> A relation has a header (definition) and set of tuples (content). If we
> look inside the relation header we find....
> by golly!.... a set of attributes!
>
> so
>
> PROJECT spaceof(<relation>) spaceof (<set of attributes>) -> PROJECTION
>
> is really more like
>
> PROJECT spaceof(attributeset(<relation>) spaceof(<set of attributes>) ->
> PROJECTION.
>
> PROJECT is just a function that returns a function. Spaceof is just a
> function that makes a space out of a set of attributes.
> Attribute set is just a function that pulls the attribute set out of the
> relation definition.
>
> Then
>
> PROJECTION <relation> -> <relation>
>
> is something we can look at in terms of closure.

The above is perhaps a better way of saying what I meant when I suggested that perhaps PROJECT is actually a powerset of unary operations. Received on Fri Aug 18 2006 - 08:51:46 CDT