Re: What to call this operator?

"Jon Heggland" <heggland_at_idi.ntnu.no> wrote in message news:MPG.1d2b3321b5ca59e19896b7_at_news.ntnu.no...
> In article <IpVve.12426$pa3.12390_at_newsread2.news.atl.earthlink.net>,
> david.cressey_at_earthlink.net says...
> > But seriously, I'm unable to figure out from the formal description
what it
> > is. more importantly, I'm unable to figure out what it is FOR. Not
that
> > there's anything wrong with your formal description. It's just my own
> > unfamiliarity with it that causes problems.
>
> It is part of a minimalistic approach to relational algebra that is more
> geared towards logic instead of set theory. <OR> is a generalisation of
> union. If the relations have the same heading, the result is a union.
>
> I think the point is to have a counterpart to <AND> (which covers join,
> product, intersection, selection and extension) that covers union, but
> places no restrictions on the types of the operands, and has simple
> logic-based semantics.
>
> > How does <OR> differ from "full outer join"?
>
> It is more like outer union than outer join. It has clearly defined
> semantics, and there are no nulls. It is, however, possibly infinite.
> --
> Jon

Thanks Jon. I'm starting to get the picture, however dimly. <AND> and <OR> are both subsets of the cartesian product of a, b, and c. Because of the way they are both defined, the linkage domain, b, follows rules for a "natural join", at my level of abstraction (which is less than this formalism).

and now I'm starting to fill in the gaps. if A is in a, B is in b, and C is in c then

A, B, C is in S <AND> T iff A,B is in S and B,C is in T. A, B, C is in S <OR> T iff A,B is ion S or B,C is in T.

Is this right? Is it complete? Received on Tue Jun 28 2005 - 15:24:34 CEST