4.2 The Resource Description Framework 71
mation in many other ways. The document shown in figure 4.5 is equivalent.
Both representations have the same RDF graph, shown in figure 4.6.
Figure 4.4 Part of a Medline citation written using RDF.<MedlineCitation Owner="NLM" Status="Completed"
MedlineID="99405456" PMID="10476541"
ArticleTitle="Breast cancer highlights">
Figure 4.5 Part of a Medline citation written using RDF. Although it looks different,
the information is the same as that in figure 4.4.
In an XML DTD or schema one can restrict the content of an element.
The analogous kinds of restriction in RDF are called domain and range con-
straints. Adomain constraintof a property restricts the kinds of resource that
can be the subjects of statements using that property. Arange constraintre-
stricts the kinds of resource that can be objects of that property. For example,
theDateCreatedproperty is only allowed to link aMedlineCitationto
aDate. An RDF ontology would state this as follows: