Re: Representing NULL in RDF

Right, but you have used out of band information to know that everyone 
has an age.  No automated process could know that.  null in SQL only 
indicates the absence of information, and that is most naturally 
indicated in RDF by the absence of a triple, just as the RDB-to-RDF 
Direct Mapping produces.  if other domain specific information is known, 
such as the fact that everyone has an age, then that information can be 
represented by additional triples that are implied by the existence of 
the row in the table -- not by the null value in the column.

David

On 06/12/2013 03:13 PM, Juan Sequeda wrote:
> 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 <http://www.juansequeda.com>
>
>
> On Wed, Jun 12, 2013 at 11:08 AM, Tim Berners-Lee <timbl@w3.org
> <mailto: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
>     <mailto: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
>
>

Received on Wednesday, 12 June 2013 20:07:20 UTC