Re: A different definition of MINUS, Part 1

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.


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