Re: What to call this operator?
Date: Tue, 28 Jun 2005 11:13:23 +0200
Message-ID: <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.
-- JonReceived on Tue Jun 28 2005 - 11:13:23 CEST