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Home -> Community -> Usenet -> comp.databases.theory -> Re: All hail Neo!
Bob Badour wrote:
> Frank Hamersley wrote:
>> Bob Badour wrote: >>> Frank Hamersley wrote: >>>> Bob Badour wrote: >>>>> Frank Hamersley wrote: >>>>>> Bob Badour wrote: >>>> [..] >>>>>>> In that line of thought, here's an interesting question that Date >>>>>>> et al have posed before to the n-VL folks: >>>>>>> >>>>>>> If "exists but empty" is true and "doesn't exist" is false, what >>>>>>> is null? >>>>>> >>>>>> Neither and both! >>>>> >>>>> I find that sort of handwaving to be a complete non-answer. >>>> >>>> I suspect you are wearing the darkly tinted glasses of >>>> preconception. Whilst I was trying to show a little wit, the current >>>> 3VL state of affairs still seems to me to fit that description. >>>> >>>>> A much more intellectually honest reply would be: "I don't know." >>>> >>>> Not from this black duck (on this occasion)! >>>> >>>>> or "Null has no similar analog in set theory." >>>> >>>> I wasn't comparing/contrasting the RM with set theory. Perhaps for >>>> you it is implicit? >>> >>> No, it is quite explicit. >> >> I am quite interested in exploring this area so please bear with me as >> my analysis is heavily tainted with practical experience and less >> (recently) so with formal theoretical studies. My response may be >> somewhat affected by the serial reading and responding to your post >> and I never feel comfortable with long posts when it finally is time >> to click send! >> >>> Relations are sets. >> >> ALL relations are sets?
OK - I presume the dichotomy is practice (SQL) vs theory (RM) in that with the former tables can exist with repeated rows but in the latter it presumes in a relation all tuples are distinct?
>> My gut feeling is that this is not true. I >> recognise that I hold to this because I view it from the damaged goods >> position (aka SQL) or the RM as propounded by Codd, rather than from a >> sounder theoretical starting point.
I'm not well read enough to be able to hazard answers to these questions. I will does some more reading on these topics after I get off Highway #1.
>> I understand this puts us at odds >> from inception. From there I expect anything brought to this issue by >> theory will be elegant, symmetrical _and_ compelling.
Relations it seems are E,S+C until missing information is added to the mix. Bags and nulls are quite apparently not but from where I am now the null at least seems the lesser of evils viz my post in the "beautiful mind" thread.
>> [2nd editing] >> Having read the texts, below I felt inclined to retract the above >> para, but then decided to leave it so you could form your own view on >> how wacky my thinking might be. That said perhaps the fact that nulls >> do exist in the RM today taken with your confirmed view that they >> shouldn't (from a theoretical basis) is implied confirmation of my >> rather unlikely original thought...he says stepping back two full >> paces - did I write that?
Sure is - in biological terms noting that the bastard child SQL inherits only half its genome from the known parent (RM) then the question is - is it a meiotic mutation or a pre-existing recessive allele now homozygous and perhaps deleterious?
>>> Relational algebra is the equivalent of set theory, and relational >>> calculus is the equivalent of predicate calculus. >> >> I have no beef here.
By logical identity I take it you mean the distinctness of each tuple?
>>> Thus, the equivalence of dee and true and of dum and false are very >>> important. And the question of what relation value equates to null is >>> a very good question. >> >> Lets put that aside until the first question has been addressed.
Yep - although there probably no debate to be had once the various and varying starting points are determined.
>>>>> True and 1 both have the exact same analog in set theory. False and >>>>> 0 both have the exact same analog in set theory. >>>> >>>> Perhaps but insistence on a parallel form for the RM does not seem >>>> to lead anywhere practical*...FWICT. >>> >>> Are you suggesting that query transformations lack practical benefits? >> >> No sure where you are going here, can you be specific or illustrate?
OK. Obviously useful for the theorist to explore stuff (apols to Alexi).
>>>>> This has a certain elegance and symmetry.
>>>>
>>>> I agree that and readily subscribe to that in my own endeavours.
>>>>
>>>>> In canonical form:
>>>>>
>>>>> {} = 0 = false
>>>>> {{}} = 1 = true
>>>>>
>>>>> What is the similar analog for null?
>>>>
>>>> My prior knowledge of your/the notation is non existent but I can
>>>> prolly deduce its intent. So having a stab at it how about ...
>>>>
>>>> {}{} = null
>>>
>>> {} is the empty set and is the set with cardinality 0.
>>> {{}} is the set containing an empty set and is the canonical form of
>>> all sets with cardinality 1.
>>> {{},{{}}} is the canonical set with cardinality 2.
>>> http://www.math.psu.edu/simpson/papers/philmath/node17.html
>>
>> Interesting read. However it seems the jury is not even cloistered
>> yet! I quote "The implications of this idea are not yet fully
>> understood and are a topic of current research". Of course, this is
>> from a web page ;-)
Which I find (philosophy) much more interesting these days, although much of it beyond me as a casual investigator.
>>> One can continue in this vein until Goedel stomps on one. >>> http://en.wikipedia.org/wiki/G%C3%B6del%27s_incompleteness_theorems >> >> OK - it reminds me of a statement made in undergrad science that "you >> don't prove a hypothesis, you can only disprove it". Of course Godel >> is more complete than that.
>>> With respect to {}{} either it is completely meaningless, or perhaps
>>> you intend the juxtaposition of two sets to mean conjunction or
>>> disjunction, in which case:
>>> {}{} = {} = false
>>
>> It was just a wild punt - the intent was to suggest that two empty
>> sets were distinct and that their intersection was indefinable.
So {1} and {1} are not distinct when conceptually imagined in an OO instance frame of mind? Thus it would take something like {1,2} and {1,3} at which point the sets are distinct? In this case are the 1's distinct?
> However, a single attribute
> within a single tuple within a single relation variable has a unique
> logical location.
True.
> How then can null equate to two locations?
I'm guessing a bit here about your question - I take it to mean {1,null} and {1,null} should not be seen as distinct but nVL insists they are?
> What then
> does it mean when the null appears in a derived value which has no
> location at all?
But the RM proscribes every value (derived or not) has a location does it not - that is putting aside the stuff about nameless or duplicated attributes etc?
>>> If null and false are the same, we can do away with null. >> >> They aren't but I guess it may be feasible to do away with null in the >> truth table ie. null implies false. I expect you would still need its >> physical presence in the manifestation of tables/relations.
No, I don't think it does. As you say it is not {} and {null} does not equal {null} - this is as I said before a paradoxical state of affairs and clearly does not marry well with the formal approach to analysis.
[..]
>> Thanks for the pointers - admittedly I have only limited time and have >> had to skim through the material, however it was apparent that there >> remains substantial areas where consensus has not been achieved (yet).
Which is why we two are assembled here today - not enough of us for a riot however!
>>>> * (for another post perhaps) I don't see any great leap forward in >>>> the aspects of TTM that address the extinction of nulls. >>> >>> If nulls cause great damange without serving any particularly useful >>> purpose, why should one address their extinction? >> >> No quite sure what your thrust is here, perhaps you would like to edit >> it given the damage (sic) it has suffered? In advance of that, I can >> state that I don't hold that nulls cause _great_ damage (although >> neither does Semtex in capable hands for a proper purpose) and I >> certainly believe (at this point) that they do serve a purpose.
I don't need them to postulate it as I've seen it first hand. My opinion of Date mainly and Darwen to a much lesser extent (more by association than real research) is that whilst they have taken a pot shot at a low flying albatross, they haven't come up with anything even close to better. Being too tight to pay them for their various treatises means that I prolly have limited information to base that view upon but what I have seen doesn't thrill me one bit. Hence my 155 mm post on the BM thread.
Given the lesser evil option I expect to rely on skilled practitioners to mitigate the risk of that damage arising. Sadly they don't grow on trees!
> Nulls attempt to address missing information but do so with no basis in
> theory. I suggest that the null elixir only offers the illusion of power:
> http://www.cs.utexas.edu/users/EWD/transcriptions/EWD08xx/EWD898.html
I understand your view. For the record mine is that null is no elixir - more like Cod Liver Oil and there ain't no illusions when swallowing that stuff.
>> As to >> Date and Darwens attempt to remove nulls as shown in the Tutorial-D >> slides it didn't seem to meet the elegance or compelling stature I >> mentioned before.
That is how I would characterise "D". On the theory side I am in a prolonged contemplation on the missing dimension in SQL (as compared to the RM) which is the temporal capability. I have made what I consider to be some progress but it is premature to discuss publicly lest I end up wearing lots of egg, suffice to say the null thang has a parallel in temporal matters. BTW I don't rate Date & Darwens offering on this either, nor FWIW much of the Snodgrass et al stuff that preceded them.
Cheers, Frank. Received on Tue Apr 25 2006 - 07:05:24 CDT
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