Re: more on delete from join

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

x v y = u
x ^ y = w

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

x = f(u,w)
y = g(u,w)

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