Re: A different definition of MINUS, Part 1

Brian Selzer wrote:
> 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.
> ...

I's rather say he was talking about the scope of a complement, a standpoint that seems more useful to me.

> 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.
>

You are really talking about language implementation and its various psychological aspects, introducing all kinds of notions I don't think are important to the logical question of equations that represent the differences between certain relations. I'm only talking about the A-algebra's logical implications in the RM. You may want me to talk about language but I won't, at least, not much. Received on Wed Dec 17 2008 - 15:23:55 CET