Re: Representing NULL in RDF

I heard somebody saying mapping from RDB to RDF? :)

In the RDB2RDF Direct Mapping [1], we do not generate a triple for null
values.

We also studied the direct mapping in the case that there are null values
in our WWW2012 paper [2]

[1] http://www.w3.org/TR/rdb-direct-mapping/
[2] http://www2012.wwwconference.org/proceedings/proceedings/p649.pdf

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


On Tue, Jun 4, 2013 at 5:45 AM, Tim Berners-Lee <timbl@w3.org> wrote:

>
> On 2013-06 -03, at 22:39, Jan Michelfeit wrote:
>
> Hi,
> thank you all for your answers.
>
>
>
> ... One "represents" a null by failing to include the relationship
>
> ... RDF semantics make no assumptions about what the absence of a
> proposition/statement means
>
>
> I agree. The question was actually about *distinguishing* between the
> mentioned cases.
>
>
> Well, then you are not mapping from RDB to RDF.
> As in the database the null didn't tell you which of your case
> it is, of your options
>
> - value not applicable (the attribute does not exist or make sense in the
> context)
> - value uknown (it should be there but the source doesn't know it)
> - value doesn't exist (e.g. year of death for a person alive)
> - value is witheld (access not allowed)
>
>
> You can have out of band, outside the table, information to tell you.
> You can rely on assumptions you can make in given applications.
>
> Both of which you can do with graphs too in *exactly and precisely *
> *the same way.*  You can have implicit or explicit assumptions
> about what the absence of a statement means.
>
> So basically IMHO a null maps exactly to no triple.
>
> Tim
>
>