Re: Notions of Type
From: J M Davitt <jdavitt_at_aeneas.net>
Date: Fri, 18 Aug 2006 02:46:40 GMT
Message-ID: <kS9Fg.77099$Eh1.22703_at_tornado.ohiordc.rr.com>
> ...
>
>
>
> Earlier, Bob B said about the "set of attributes": "It is a relation of
> degree 1 and cardinality N representing a set of N attribute names".
> Maybe you are right and saying that isn't necessary, but his stance
> seems to me to obviate the question.
>
> (I don't know if it matters that if his approach is right, projection
> wouldn't depend on whatever the name of the single attribute is - I
> presume that it would have the same name and type as all headers in the
> db, eg., just another axiom in the form of the catalog.)
>
> p
Date: Fri, 18 Aug 2006 02:46:40 GMT
Message-ID: <kS9Fg.77099$Eh1.22703_at_tornado.ohiordc.rr.com>
paul c wrote:
> 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
In some regard, BB's relation is just fine. In D+D's works, a
set of attributes wouldn't be a relation value: no heading.
This is one of the areas, I think, where we've been writing
"relation" and, in the D+D sense, some think "relation values"
and others, in the RA sense (to the extent I understand it),
think "relations."
There are differences in the definition of operations in the
two; is this one instance?
[Or, it could just be me...]