- Blank node
In RDF, a blank node or anonymous resource or "bnode") is a resource, or node in an RDF graph, which is not identified by a
URI . A blank node can be used as subject or object in an RDF triple.Example
"John has a friend born the 21st of April" can be written with two triples linked by a blank node representing the anonymous friend of John.
ex:John foaf:knows _:p1 _:p1 foaf:birthDate 04-21
The first triple reads "John knows p1". The second triple reads "p1 is born on April 21st"
ex:John is a named resource, which means this resource is absolutely identified by the URI obtained by replacing the ex: prefix by the XML namespace it stands for, such as
http://example.org/Person#John ._:p1 represents John's anonymous friend, not identified by a URI. One can know by the semantics declared in the
FOAF vocabulary that the class of _:p1 is foaf:Person.RDF-XML Notation
In RDF-XML syntax a blank node can be represented by nested elements, such as the following.
If the same blank node is used more that once in the same RDF graph, it can be identified by a rdf:nodeID attribute. This identification is limited to the local graph. For example to express that John and Mary have a common friend, one can write.
Classical use cases
Representation of complex data
A blank node can be used to indirectly attach to a resource a consistent set of properties which together represent a complex data, such as a postal address. The different fields of the complex data are represented as properties attached to the blank node. For example in the RDF
VCard vocabulary one will write.
= Anonymous classes in OWL=The ontology language OWL uses blank nodes to represent anonymous classes such as unions or intersections of classes, or classes called restrictions, defined by a constraint on a property.
For example to express that a person has at most one birth date, one will define the class "Person" as a subclass of an anonymous class of type "owl:Restriction". This anonymous class is defined by two attributes specifying the constrained property and the constraint itself (
cardinality > 1)
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