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Home -> Community -> Usenet -> comp.databases.theory -> Re: How can I proove associtivity of natural in relational algebra?
Mikito Harakiri wrote:
> Filter911 wrote:
> > Mikito Harakiri wrote:
> > > Filter911 wrote:
> > > > Can someone give me a link for a full proof or something?
> > >
> > > Given relations A,B, and C, expand each realtion into a (possibly
> > > infinite) relations A', B', and C' with the same set of attributes
> > > (which is the union of the attribute sets for A, B, and C). Then, the
> > > join of A, B and C is the intersection of A', B', and C'. Intersection
> > > is associative.
> >
> > What do you mean by "expand each relation"?
>
> "Extend", is the opposite of "project". E.g. the relation with a single
> attribute
>
> A = {(a=1), (a=2)} is exended to the attribute set {a,b} with the
> domain of attribute b being {7,8,9} as
Extanding using what? Using B or C? or some new relation? any full
example or link?
If I use the union of the three then A=B=C and that's a prviate case
> A' = {(a=1, b=7),(a=1, b=8),(a=1, b=9),(a=2, b=7),(a=2, b=8),(a=2,
> b=9))}
Received on Tue Nov 22 2005 - 05:40:55 CST