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. (01)
Regards (02)
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/ (03)
This email originates from Information Junction Ltd. Registered in England
and Wales No. 6632177.
Registered office: 2 Brookside, Meadow Way, Letchworth Garden City,
Hertfordshire, SG6 3JE. (04)
> -----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
>
> _________________________________________________________________
> Message Archives: http://ontolog.cim3.net/forum/ontolog-forum/
> Config Subscr: http://ontolog.cim3.net/mailman/listinfo/ontolog-forum/
> Unsubscribe: mailto:ontolog-forum-leave@xxxxxxxxxxxxxxxx
> Shared Files: http://ontolog.cim3.net/file/ Community Wiki:
> http://ontolog.cim3.net/wiki/ To join: http://ontolog.cim3.net/cgi-
> bin/wiki.pl?WikiHomePage#nid1J
>
>
> _________________________________________________________________
> Message Archives: http://ontolog.cim3.net/forum/ontolog-forum/
> Config Subscr: http://ontolog.cim3.net/mailman/listinfo/ontolog-forum/
> Unsubscribe: mailto:ontolog-forum-leave@xxxxxxxxxxxxxxxx
> Shared Files: http://ontolog.cim3.net/file/ Community Wiki:
> http://ontolog.cim3.net/wiki/ To join: http://ontolog.cim3.net/cgi-
> bin/wiki.pl?WikiHomePage#nid1J
> (05)
_________________________________________________________________
Message Archives: http://ontolog.cim3.net/forum/ontolog-forum/
Config Subscr: http://ontolog.cim3.net/mailman/listinfo/ontolog-forum/
Unsubscribe: mailto:ontolog-forum-leave@xxxxxxxxxxxxxxxx
Shared Files: http://ontolog.cim3.net/file/
Community Wiki: http://ontolog.cim3.net/wiki/
To join: http://ontolog.cim3.net/cgi-bin/wiki.pl?WikiHomePage#nid1J (06)
|