Re: Representing NULL in RDF

It depends.

If I have a NULL for the column age, we can all assume that everybody has
an age (there exist an age), but I don't know what it is. So it would be
"safe" to have  <x> :age _:age

Juan Sequeda
+1-575-SEQ-UEDA
www.juansequeda.com


On Wed, Jun 12, 2013 at 11:08 AM, Tim Berners-Lee <timbl@w3.org> wrote:

>
> On 2013-06 -10, at 19:48, Steve Harris wrote:
>
> > On 2013-06-09, at 20:36, Pat Hayes <phayes@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.
>
>
> Surely a null in  an RDF database conveys no information about the
> thing, unless you have out of band knowledge.
> If you have NULL for a cellphonenumber, then that normally means no one
> stored a cellphone number,
> but it doesn't mean that there is a cellphone whose number is unknown.
>
> A blank node means "There exists one."   As in "This person has some
> cellphone number".
> which is very different.
>
> Nulls should be converted.
>
> Tim
>