There can be a Subject or an Object that has no URI associated with it and than it is a blank/empty node. It simply denotes the existence of some individuals that has specific properties or that has specific individual attributes, but it is note denoted which individual it is, it has no name. Sometimes it is very convenient to make a statement about that exists without telling which individuals directly are addressed with this information. For this case Blank Nodes are necessary.
Blank nodes do not have a name and potential are not referencable.
Multi valued Relations raise conceptional problems regarding uniqueness, e.g. events that take place in regular intervals. An ordering or selection can be create by introduction additional Blank Nodes. Blank Nodes can for instance represent a place and a time that combines in a unique way.
Blank Nodes can be introduced to represent multi valued Relationships and especially for resources that do not need a name (auxiliary nodes). In general Blank Nodes make some statement of existing of something that has specific properties.
In XML for this a new attribute a so called rdf:parseType need to be introduced.
<xml version="1.0" encoding="utf-8"> rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-synthax-ns#" xmlns:event="http://denigma.de/events#" >]
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-nx#>] . @prefix prof: <http:denigma.de/events/> . <http://denigma.de/events/event/meeting5>] events:takesPlace [ events:hasDate "Tue 13.30-15.00"; events:hasRoom "HS3" ].
Sometime it is important to dereference the node. It must sometimes be able to be addressed from outside. In such a case the Blank Node can have a name.
<rdf:Description rdf:about="http://denigma.de/events">] <events:takesPlace rdf:nodeID="ID1" /> </rdf:Description> <rdf:Description **rdf:nodeID="ID1"**> <event:hasDate>Tue 13.30-15.00</event> <event:hasChannel>chat5<event:hasRoom> </rdf:Description>
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-nx#>] . @prefix event: <http://denigma/events/>] <http://denigma.de/event/meeting5>] event:takesPlace _:ID1 . _:ID1 event:hasDate "Tue 13.30-15.00"; event:hasChannel "chat5" .
From now on anywhere everywhere in this RDF document one is able to address this node.
Thus, Blank nodes are to referenced or not to referenced things that do not have an existince in the real world or outside the RDF document, but nevertheless can be referenced if given a name.