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From: jsmith@king.mcs.drexel.edu
Subject: Differences
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Date: Tue, 30 Mar 93 09:45:40 EST
From: jsmith@king.mcs.drexel.edu


How does one compute DIFFERENCES in postgres? What I mean is, how do
you compute A-B, where A and B are relations?
(A-B is the tuples in A that are not in B). As I understand it, the 
relational paradigm requires one to be perform this operation. It has
applications to universally quantified queries
(In a database of magazine-subscribers and publishers the query
"Which subscribers get ALL of their subscriptions from a certain
publisher?" requires forming differences, in this sense).


_____________________________________________________________________________
Justin R. Smith                                        Office: (215) 895-1847
Department of Mathematics and Computer Science           Home: (215) 446-5271
Drexel University                                         Fax: (215) 895-2070
Philadelphia, PA 19104  (USA)