Zitat von Steve Harris <steve.har...@garlik.com>:
On 2013-06-09, at 20:36, Pat Hayes <pha...@ihmc.us> wrote:
...
- value uknown (it should be there but the source doesn't know it)
Actually that piece of information could be written down in a RDF
Schema graph like this:
It can be written far more simply in RDF just by using a blank node:
:a :p _:x .
Yes, a blank node is probably the closest thing to a SQL NULL in RDF.
Though it would be safer/clearer to write:
:a :p [] .
We are not talking about serializations. ;)
To emphasise the (NULL = NULL) → FALSE behaviour, which is key to
the uses of NULL in SQL.
Excellent point!
However, the convention in RDF is just to omit the property for that
subject where something is unknown - as others have said.
Jan asked about how to distinguish between his four cases which are
indeed slightly different to each other.
Omitting a triple has therefore a different quality than telling the
existence of the property of this very subject. So, there is more
information (existence) than when omitting the whole triple.
--
Sven R. Kunze
Chemnitz University of Technology
Department of Computer Science
Distributed and Self-organizing Systems Group
Straße der Nationen 62
D-09107 Chemnitz
Germany
E-Mail: sven.ku...@informatik.tu-chemnitz.de
WWW: http://vsr.informatik.tu-chemnitz.de/people/kunze
Phone: +49 371 531 33882
