# Re: header part of the value?

Date: Thu, 28 Feb 2008 11:35:45 -0800 (PST)

Message-ID: <50e25168-bbc3-4297-8772-00332caff67a_at_s37g2000prg.googlegroups.com>

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:

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**> > On Feb 28, 12:03 am, Jan Hidders <hidd..._at_gmail.com> wrote:
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**> > > 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:
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**> > > > > 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.
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**> > > > 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}.

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 ..."

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. Received on Thu Feb 28 2008 - 20:35:45 CET