So it sounds like NULL should really be refactored, even in existing
database models,
into four or five possible values,
and maybe keep NULL for just one of them. (01)
I would use it for scenario 1 - No value is meaningful. (02)
That "Humpty Dumpty" reference is great, btw. (03)
--
Edmon (04)
On Tue, Jan 22, 2013 at 11:51 AM, Barkmeyer, Edward J
<edward.barkmeyer@xxxxxxxx> wrote:
> At the risk of sounding like an old pedagogue, you might consider reading a
>Ted Codd paper from about 1980 on the possible interpretations of Null.
> My recollection is that he identified at least 4 possible interpretations:
> 1) No value is meaningful (the property does not apply to things with
>whatever other properties are documented)
> Example: a person whose marital status is "single" does not have a value
>for "spouse"
> 2) No value in this instance (the thing does not have an instance of this
>property)
> Example: a person may have no value for "mobile phone number" (by not
>having the service)
> 3) The value should exist but is not known (no one entered this datum)
> Example: a missing "date of birth"
> 4) Whether there is a value is unknown
> And there is a further state that we now sometimes see:
> 5) The value has been redacted. It may be in the database, but it is not
>in the query result, because the requester does not have the authority to ask
>for it (in this case).
>
> And there may well be others. "Null" is a Humpty Dumpty word.
>
> -Ed
>
> 'When I make a word do a lot of work like that,' said Humpty Dumpty,
> 'I always pay it extra.' ... Ah, you should see 'em come round me of
> a Saturday night,' Humpty Dumpty went on, wagging his head gravely
> from side to side: 'for to get their wages, you know.'"
> -- Lewis Carroll (C.L. Dodgson), "Through the Looking Glass"
>
>> -----Original Message-----
>> From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx [mailto:ontolog-forum-
>> bounces@xxxxxxxxxxxxxxxx] On Behalf Of Matthew West
>> Sent: Tuesday, January 22, 2013 4:45 AM
>> To: '[ontolog-forum] '
>> Subject: Re: [ontolog-forum] Knowledge graphs by Google and Facebook
>>
>> Dear Bob,
>> That is precisely the problem with NULL. You don't know if it means not
>> known, or does not exist. In principle it could mean either in any situation.
>>
>> Regards
>>
>> Matthew West
>> Information Junction
>> Tel: +44 1489 880185
>> Mobile: +44 750 3385279
>> Skype: dr.matthew.west
>> matthew.west@xxxxxxxxxxxxxxxxxxxxxxxxx
>> http://www.informationjunction.co.uk/
>> http://www.matthew-west.org.uk/
>>
>> This email originates from Information Junction Ltd. Registered in England
>> and Wales No. 6632177.
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>> Hertfordshire, SG6 3JE.
>>
>>
>>
>> > -----Original Message-----
>> > From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx [mailto:ontolog-forum-
>> > bounces@xxxxxxxxxxxxxxxx] On Behalf Of Natale, Bob
>> > Sent: 22 January 2013 08:38
>> > To: [ontolog-forum]
>> > Subject: Re: [ontolog-forum] Knowledge graphs by Google and Facebook
>> >
>> > I am sure there is a really good reason why none of the experts on
>> > this
>> thread
>> > have mentioned the "NULL' value in RDBMS practice, but for the life of
>> > me
>> I
>> > cannot guess it. It's simply not true that "not found" must mean "not
>> exists"
>> > in RDBMS (nor SQL) -- NULL can be set as an explicit or default value ...
>> the
>> > meaning of which is more akin to "not yet specified (and therefore not
>> > yet known, one way or the other)". That seems more OWA than CWA, if I
>> > am following you good people correctly.
>> >
>> > Cheers,
>> > BobN
>> >
>> > -----Original Message-----
>> > From: ontolog-forum-bounces@xxxxxxxxxxxxxxxx [mailto:ontolog-forum-
>> > bounces@xxxxxxxxxxxxxxxx] On Behalf Of John F Sowa
>> > Sent: Tuesday, January 22, 2013 2:35 AM
>> > To: ontolog-forum@xxxxxxxxxxxxxxxx
>> > Subject: Re: [ontolog-forum] Knowledge graphs by Google and Facebook
>> >
>> > William and Kingsley,
>> >
>> > I'll be traveling for the rest of this week, so this is my last note
>> > on
>> this
>> > thread -- at least until next week.
>> >
>> > WF
>> > > So, this is what I think I have learned from this discussion, that I
>> > > can
>> > > apply:
>> > >
>> > > "The only known semantics that makes SQL and many other
>> > > computer-usable (for the things we want computers to do) -languages
>> > > work
>> is
>> > something
>> > > called the "Closed World Assumption". This so-called "assumption" is
>> > > in fact obviously false, so when we go about using the the results
>> > > of queries in our programs and interpreting them as people, we need
>> > > to be very careful to take this weakness of the computer languages
>> > > into
>> account.
>> >
>> > Actually, the theoretical issues are more subtle, and the practical
>> > issues
>> are
>> > simpler.
>> >
>> > The CWA is the assumption that makes SQL, Prolog, and many other
>> > systems amenable to classical semantics for first-order logic.
>> > Another name for "classical" is "Tarski-style", since Alfred Tarski
>> > stated
>> the
>> > method, called model-theoretic semantics, around 1930.
>> >
>> > There is also more recent work on nonmonotonic reasoning, which deals
>> > with
>> the
>> > Open World Assumption (OWA) and many related issues, including default
>> logic,
>> > negation as failure, circumscription, and belief revision. There is a
>> large
>> > "cottage industry" for producing papers and dissertations about these
>> issues.
>> >
>> > The simple solution for practical purposes is the one you suggested:
>> > just recognize that nearly all databases are incomplete (in the sense
>> > that much of the expected information is missing). Then remember that
>> > 'not' in
>> the
>> > query language means 'not found'.
>> >
>> > As long as you remember those principles and you write suitable
>> > exception handlers for dealing with the inevitable failures that
>> > arise, your systems will work reasonably well for most practical
>> > purposes. When they fail,
>> you
>> > can do what Microsoft does -- issue patches every Tuesday.
>> >
>> > KI
>> > > A DB2 relational dbms that in version 10 is enhanced with RDF data
>> > > model storage and SPARQL query language support
>> >
>> > Yes. Oracle also handles RDF and SPARQL. The vendors who sell graph
>> based
>> > systems support SQL, and the relational vendors support SPARQL.
>> >
>> > The conceptual schema proposals developed by the DB community in the
>> > 1970s (and '80s and '90s) were intended to support interoperability.
>> > They tried to make a clean separation between the logic and the
>> implementation
>> > details. The major obstacle was the vendors who didn't want
>> interoperability
>> > with products sold by their competitors
>> >
>> > John
>> >
>> >
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