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4.2 The Resource Description Framework 67



  1. Open vs. closed worlds.When evaluating queries it is important to know
    whether the database is complete or “closed.” Whether based on the re-
    lational model or on XML, databases differ markedly from RDF in this
    regard: RDF is open while databases are closed.
    RDF refers to relationships asproperties, which includes both attributes and
    the relationships specified using striping. When a property has a simple data
    value, one can use either an attribute or a child element to express it. Both of
    these have the same meaning in RDF, while they have very different mean-
    ings in XML. When one is designing a DTD or schema, it is necessary to make
    a choice about how relationships will be expressed. The question of whether
    one should use an attribute or a child element is one of the most common de-
    cisions one must make. Some DTDs, such as the one for Medline, primarily
    use child elements. Other DTDs, such as CML, prefer to use attributes. RDF
    eliminates the need for this choice.
    The mathematical model that defines the semantics of RDF is a graph
    (W3C 2004a), consisting of nodes and links, much like the one used by XML
    infosets. However, RDF graphs can be arbitrary graphs, while XML infos-
    ets are strictly hierarchical, starting from the root. In addition, as mentioned
    before, all RDF links are labeled, while XML infosets only label the attribute
    links. An RDF node is a resource, and an RDF link is labeled by a prop-
    erty. Resources are classified using RDF classes, and every resource is an
    instance of at least one RDF class. Classes can be related to one another
    by the RDF property namedrdfs:subClassOf. When an RDF class is a
    subclass of another, then the instances of the subclass are automatically also
    instances of the other class. ThesubClassOfrelationship defines the class
    hierarchy of an RDF ontology. Properties can be related to one another by
    rdfs:subPropertyOf, and it has a similar meaning tosubClassOf,but
    it is much less commonly used.
    Classes and properties are the fundamental organizational entities of RDF.
    Consider the first part of the example in figure 1.14:






format="GENBANK"/>
entry="V00565"/>
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