Re: header part of the value?

From: Jan Hidders <hidders_at_gmail.com>
Date: Thu, 28 Feb 2008 13:02:27 -0800 (PST)

On 28 feb, 20:35, Tegiri Nenashi <TegiriNena..._at_gmail.com> wrote:
> On Feb 28, 8:37 am, Jan Hidders <hidd..._at_gmail.com> wrote:
>
>
>
> > On 28 feb, 17:27, Tegiri Nenashi <TegiriNena..._at_gmail.com> wrote:
>
> > > On Feb 28, 12:03 am, Jan Hidders <hidd..._at_gmail.com> wrote:
>
> > > > On 28 feb, 04:32, Tegiri Nenashi <TegiriNena..._at_gmail.com> wrote:
>
> > > > > On Feb 27, 7:08 pm, Marshall <marshall.spi..._at_gmail.com> wrote:
>
> > > > > > It appears to me as if you are viewing the
> > > > > > equation "x + 3 = y" as having 3 as the free variable, and
> > > > > > x and y as constants! Whereas I would consider it much
> > > > > > more natural to consider x and y as free variables, or
> > > > > > parameters, or attribute names (they are all the same)
> > > > > > and 3 and 5 as constants.
>
> > > > > Well, the case with two variables in the header is tricky. First,
> > > > > define union and join then we can talk about it:-)
>
> > > > How about: the union is the logical disjunction of the equations and
> > > > the join is the logical conjunction of the equations?
>
> > > Can you please apply your definition on the example:
>
> > > Q:
> > > x + 3 = y \/
> > > x + 5 = y
> > > R:
> > > x + y = 7
>
> > > What is the header of the join, is it {"x+...=y", "x+y=7"}, or is it
> > > {"x=...", "y=..."}. What is the header of the union?
>
> > In both cases {x,y}.
>
> Are you implying that you always extract attribute names from
> generalized header and do usual set operations and the result is
> always classic relation? This can't be because the idempotence is
> violated, for example, what would be the result of joining the Q with
> itself? I might also interpret you answer as if you don't subscribe to
> the idea of generalized relation, so that the header of Q is just
> {x,y}.

Indeed, I don't.

> Let me reiterate the generalized relation idea one more time, on a
> level perhaps more digestable for wider audience. Consider classic
> relation
>
> "The person first name is ..."
>
> Normally, we don't write the whole sentence in the relation header (we
> focus exclusively on named perspective, of course) and abbreviate it
> to just
>
> Name
> -----
> Scott
> Mike
>
> The concept of domain has been introduced to resolve questions weather
> this relation is allowed to be joined with something like
>
> "The ship name is ..."
>
> All we do when allowing generalized relations is admitting predicates
> like this:
>
> "The variable x is greater or equal than ..."
>
> and insisting that the whole sentence matters as a relation header.

• Jan Hidders
Received on Thu Feb 28 2008 - 22:02:27 CET

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