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On Wed, Sep 16, 2009 at 09:45:52AM -0700, Jeff Davis wrote:
> On Wed, 2009-09-16 at 15:11 +0200, tomas@tuxteam.de wrote:
> > One question: does the operator have to be reflexive? I.e. "A op A holds
> > for all A"?
>
> I don't think that reflexivity is a strict requirement. You could make
> this a constraint over a boolean attribute such that false conflicts
> with true and true conflicts with false. That would mean that your table
> would have to consist of either all false or all true.
Let me see whether I've understood this: more in general, op could be
"not equal" -- i.e. <>. Then, once one value was inserted into the
column, all other values would have to be equal.
[...]
> That's an interesting idea: "proximity constraint". I like it because
> (a) "proximity" might reasonably be considered a more general form of
> the word "unique", which might satisfy Peter's argument; (b) it conveys
> the use case; and (c) it sounds good.
Yes, "riding" on this geometric metaphor (distance), I was trying to
visualize relations which are symmetric but not refexive and came up
with things like
"point X is at least d1 far, at most d2 far from Y"
i.e. Y are all points which are whithin a ring centered on Y, with inner
radius d1 and outer radius d2. Special cases would be d1=0, d2>0 (that's
conventional proximity) -- but there are weirder cases, as your example
above (d2 possibly infinite, d1 small, giving a "punctured space").
> There are a couple bizarre cases where "proximity" doesn't quite fit,
> like my boolean example above, but I'm OK with that.
Hmmm.
Regards
- -- tomás
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