Re: A different definition of MINUS, Part 1

From: Brian Selzer <>
Date: Wed, 17 Dec 2008 05:16:16 -0500
Message-ID: <Hh42l.9752$>

Just some food for thought:

  1. The scope that McGoveran referred to is the set of all tuples that appear in any possible value for the relvar.
  2. When dealing with an update, there is always a before and an after, unless the update is null. If "Paul is asleep," and the update states, "Paul is awake," then which is the case? Is he asleep or is he awake? He can't be both. An expression of the algebra can only deal with one set of circumstances--not both, since the algebra is for determining what is the case rather than asserting what is the case. Each possible value for a database corresponds to a distinct logical proposition, yet only one of those propositions represents what /is/ the case. So whenever there is an update (unless it is null, of course), you are always dealing with two distinct propositions representing /what has been the case/ and /what is now the case/. I just think that ignoring before and after invites dire consequences.
Received on Wed Dec 17 2008 - 11:16:16 CET

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