Re: Representing NULL in RDF

Jan Michelfeit <michelfeit.jan@gmail.com> writes:
> I was doing some comparison of relational databases and Linked Data and ran
> into the problem of representing an equivalent of database NULL in RDF.
>
> I was surprised I haven't found any material or discussion on this topic
> (found only [1]) - is there some?. I believe it would be beneficial if this
> question was answered somewhere for future reference. I started a question on
> Stack Overflow [2] where I think it will be easier to discover and so that
> this list won't get polluted.
>
> I'm aware of the open world assumption in RDF, but NULL or a missing value can
> have several interpretations, for example:
>
> - 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)
>
> I would like to known whether there is some *standard or generally accepted*
> way of distinguishing these cases. If you have an answer, please put it on
> [2], is possible.

It's a little unclear what you could do with this. 

Say, for example, you assert that a value is withheld. Then I assert the
value. With a database, this cannot happen; either a value if null or it
is not.

Value unknown is easy. Just don't say anything. 

Value not applicable and doesn't exist, given your examples, seem the
same to me. And depends on what bit of RDF you are using. You can, for
example, assert that anything with a year of death is necessarily a dead
person. Asserting a death date for a life person will result in
contradictory information that you have to deal with in some way.

Phil

Received on Monday, 3 June 2013 15:52:40 UTC