Blank node
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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 bank node can be used as subject or object in an RDF triple.
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[edit] Example
"John has a friend born the 21st of April" can be written with two triples linked by a blank node représenting the anonymous friend of John.
ex:Jean 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.
[edit] RDF-XML Notation
In RDF-XML syntax a blank node can be represented by nested elements, such as the following.
<foaf:Person rdf:about="http://example.org/Person#John"> <foaf:knows> <foaf:Person foaf:birthDate="04-21"/> </foaf:knows> </foaf:Person>
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.
<foaf:Person rdf:about="http://example.org/Person#John"> <foaf:knows> <foaf:Person rdf:nodeID="b1"/> </foaf:knows> </foaf:Person> <foaf:Person rdf:about="http://example.org/Person#Mary"> <foaf:knows> <foaf:Person rdf:nodeID="b1"/> </foaf:knows> </foaf:Person>
[edit] Classical use cases
[edit] 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.
<rdf:Description rdf:about = "http://qqqfoo.com/staff/corky" >
...
<vCard:ADR rdf:parseType="Resource">
<vCard:Street>111 Lake Drive </vCard:Street>
<vCard:Locality>WonderCity </vCard:Locality>
<vCard:Pcode>5555</vCard:Pcode>
<vCard:Country>Australia</vCard:Country>
</vCard:ADR>
...
</rdf:Description>
[edit] Anonymous classes in OWL
The ontology language OWL uses blank nodes to represent annymous 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 bith 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)
<owl:Class rdf:about="http://example.org/ontology/Person">
<rdfs:subClassOf>
<owl:Restriction>
<owl:maxCardinality>1</owl:maxCardinality>
<owl:onProperty rdf:resource="http://xmlns.com/foaf/0.1/birthDate"/>
</owl:Restriction>
</rdfs:subClassOf>
</owl:Class>
