Re: ISSUE-41 bNode semantics: Information preservation

On 24 May 2011, at 12:00, Richard Cyganiak wrote:
> Unfortunately, we know that there cannot be a semantics preserving mapping from relational databases to RDF in the presence of NULLs. NULLs in SQL can indicate the absence of a value, and negation cannot be expressed in RDF.

Thinking more about it, this might not be true.

NULL in SQL means: “Either this has an unknown value, or it has no value.”

I'll try to restate that in logical terms.

A(?x, NULL) means: There exists some ?y such that A(?x,?y) or there exists no ?y such that A(?x,?y).

That says exactly nothing. It is trivially true.

So, not creating a triple for NULL is actually semantics-preserving. Or is it? This sounds too simple to be true, so I probably made some stupid mistake.


Received on Tuesday, 24 May 2011 11:29:46 UTC