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From: rts@unc.UUCP (Rick Snodgrass)
Newsgroups: net.database
Subject: Re: Problem with relational theory
Message-ID: <316@unc.unc.UUCP>
Date: Mon, 8-Sep-86 09:53:33 EDT
Article-I.D.: unc.316
Posted: Mon Sep  8 09:53:33 1986
Date-Received: Thu, 11-Sep-86 08:26:27 EDT
References: <595@ur-tut.UUCP> <452@hdsvx1.UUCP> <819@navajo.STANFORD.EDU>
Reply-To: rts@unc.UUCP (Rick Snodgrass)
Organization: CS Dept, U. of N. Carolina, Chapel Hill
Lines: 29

In article <819@navajo.STANFORD.EDU> avg@navajo.UUCP (Allen Van Gelder) writes:
>I disagree with Hoffman's attempted justification of the way
>Eric Carleen's query is handled.  Having a calculus-based language
>loses its point if the user has to know how the system is going
>to translate it to relational algebra, possibly incorrectly.
>
>His query, essentially:
>	retrieve t1  where t1.name = t2.name  or t1.name = t3.name
>has an obvious meaning in relational calculus, and he has a right
>to expect the answer to be based on this meaning.
>
>The correct way to translate an "or" in a selection is as a union.
>This is shown in "Principles of DB Systems," by J.D. Ullman in
>proving that relational algebra has the same power as relational
>calculus.

One must be careful in deciding what the meaning of a statement in
any language is. The best way is to go back to the formal semantics
of the language. In his book, Ullman gives the semantics for Quel
(Section 6.4). For this query, it is
	{u | (Exists t1)(Exists t2)(Exists t3)
		(R(t1) and R(t2) and R(t3) and u = t1
			and (t1[name] = t2[name] or t1[name] = t3[name])
		) }
This semantics, which supports Hoffman's reply, is independent of
the relational algebra.

Rick Snodgrass
Department of Computer Science, University of North Carolina at Chapel Hill