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 On Wed, Jun 12, 2013 at 11:08 AM, Tim Berners-Lee <[email protected]> wrote: > > On 2013-06 -10, at 19:48, Steve Harris wrote: > > > On 2013-06-09, at 20:36, Pat Hayes <[email protected]> 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 >
