On 2013-06 -10, at 19:48, Steve Harris wrote: > On 2013-06-09, at 20:36, Pat Hayes <pha...@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