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). (01)
And there may well be others. "Null" is a Humpty Dumpty word. (02)
-Ed (03)
'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" (04)
> -----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/
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>
> This email originates from Information Junction Ltd. Registered in England
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>
>
> > -----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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