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From: rts@unc.UUCP (Rick Snodgrass)
Newsgroups: net.database
Subject: Re: Problem with relational theory
Message-ID: <353@unc.unc.UUCP>
Date: Sun, 14-Sep-86 19:53:08 EDT
Article-I.D.: unc.353
Posted: Sun Sep 14 19:53:08 1986
Date-Received: Sat, 27-Sep-86 09:27:00 EDT
References: <595@ur-tut.UUCP> <452@hdsvx1.UUCP> <819@navajo.STANFORD.EDU> <316@unc.unc.UUCP> <5717@ut-sally.UUCP>
Reply-To: rts@unc.UUCP (Rick Snodgrass)
Organization: CS Dept, U. of N. Carolina, Chapel Hill
Lines: 16

In article <5717@ut-sally.UUCP> ark@sally.UUCP (Arthur M. Keller) writes:
>The problem is where the quantifiers go.  Consider:
>
>{u | (Exists t1)(R(t1) and u = t1 and
>	[(Exists t2) (S(t2) and t1[name]=t2[name]) or
>	(Exists t3) (T(t3) and t1[name]=t3[name])])}
>
>This query has ordinary union semantics, even if either S or T is
>empty.  It is not clear that when given a quantifier-free expression
>that quantifiers should always be introduced at the beginning.

The Quel semantics states very clearly that quantifiers, one per
tuple variable, are to be introduced at the beginning--see (6.5)
on page 190 in Ullman's book. Allowing them to be put in arbitrary
positions may solve this one confusion but introduces others, and
is counter to the design of Quel.