# Re: header part of the value?

Date: Thu, 28 Feb 2008 13:02:27 -0800 (PST)

Message-ID: <1a50b1d5-8691-495a-a212-cea32d25b9e3_at_s13g2000prd.googlegroups.com>

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