Re: more on delete from join
Date: Tue, 1 Sep 2009 11:15:04 -0700 (PDT)
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
> 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?
? One of the basic RL results is that it is undeconstrained! Received on Tue Sep 01 2009 - 20:15:04 CEST