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. Though it would be safer/clearer to write: :a :p [] . To emphasise the (NULL = NULL) → FALSE behaviour, which is key to the uses of NULL in SQL. However, the convention in RDF is just to omit the property for that subject where something is unknown - as others have said. - Steve -- Steve Harris Experian +44 20 3042 4132 Registered in England and Wales 653331 VAT # 887 1335 93 80 Victoria Street, London, SW1E 5JL