RE: Representing NULL in RDF

Eyup!

This is something that I've thought about quite a lot as a geographer modelling the world, society, organisations and people. I have a model that represents individuals and which ticks on a daily time step, so Juan's email is back on this topic so I thought I'd pipe up again. Some may think that everything can be defined with triples, but I think that is only the case if we are modelling in context, such as within a single frame of time. So maybe this helps and maybe not, at least it is fairly brief:

Do people have an age if they are dead? Probably they don't have an age much before they are born.

At best we have an approximate time of birth and time of conception for people.

Those that have died have a time of death. Again this is only known approximately, so ranges are important with such things.

To estimate  a person's age at a particular time there can be a calculation.

Then we can express that a person has an age at a particular time. It really only makes sense to give an age anchored to a time.

In other words, age is such a continuous variable that I don't think it really makes expressing an individuals' age without more context.

But age is also discrete. People commonly talk of age meaning age in years (after some event e.g. birth). Indeed anniversaries are important events and rules change for people depending on this and depending on other dates that pass while someone is aged in a particular age category.

My point is that to deal with age, you need something more than triples unless all your data related to a single point in time.

People are more complicated than buildings, but consider the above about age when considering a building or something even less well defined like an organisation.

I hope if nothing more that this was good food for thought.

Best wishes,

Andy
http://www.geog.leeds.ac.uk/people/a.turner/

From: Juan Sequeda [mailto:juanfederico@gmail.com]
Sent: 12 June 2013 20:13
To: Tim Berners-Lee
Cc: Steve Harris; Pat Hayes; Sven R. Kunze; public-lod
Subject: 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<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