# Re: more on delete from join

Date: Tue, 1 Sep 2009 11:15:04 -0700 (PDT)

Message-ID: <e60aec47-04b2-47f1-943e-f9a5c882816a_at_13g2000prl.googlegroups.com>

On Aug 31, 9:59 am, "Walter Mitty" <wami..._at_verizon.net> wrote:

*> ... Is the same as a relation on x and y such that the equation is true. Of
**> course if x and y are defined as integers, this will give a different
**> relation than if x and y are defined as reals (or some finite subset of
**> reals) so a system of equations is a special case of a system of
**> relations.
**>
**> x + y = 8
**> x - y = 2
**>
**> Is a system of equations and therefore a system or relations. Now the
**> intersection of thses two relations is the single point, (5, 3) or, if you
**> prefer (x=5, y=3).
**> Solving the system boils down to finding the intersection of two relations.
**> A system of equations can be overconstrained, and have no solutions (the
**> intersection is the empty set) or underconstrained
**> (the intersection is a set with many elements).
**>
**> Now if we switch over to
**>
**> A JOIN B = C
**> ...
*

Well the analogy with linear system of equations

x + y = 8

x - y = 2

can be extended a little more. If (natural) join is analogus to '+' then (inner) union is somewhat analogos to minus. Perhaps we can update a system of views that consisting of join and union viewes?

OK, here is a simpler question: is the system

underconstrained, or it could be solved in terms of x and y

? One of the basic RL results is that it is undeconstrained! Received on Tue Sep 01 2009 - 20:15:04 CEST