Re: Representing NULL in RDF

David,

You are right and I should have clarified that.

First of all, nulls in rdbms are... hairy.

If you have outside knowledge, which could be given to an automated system,
then generating a triple with a blank node *could* be the right thing to
do. But it all depends on the ... semantics :) Unfortunately, at least in
RDBMSs, a lot of the semantics are in the devs head or in the application
code.

I'm jumping into this thread right in the middle so apologies if I'm
stating something that was already said or obvious.

Just my 2cts.



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


On Wed, Jun 12, 2013 at 3:06 PM, David Booth <david@dbooth.org> wrote:

> 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
>>
>>
>>