1.6 Relationships 27
Figure 1.15 The use of references to specify a bond between two atoms in a
molecule. The arrows show the atoms that are being referenced by the bond element.
meaning. A relationship specified with either striping or a reference forms
a statement. For example, in figure 1.14 there is the statement “The human
insulin gene is cited by db entry 80129725.” Both striping and references help
organize XML documents so that relationships are explicit. They contribute
to the goal of ensuring that data are self-describing. References are com-
monly used in bioinformatics ontologies, but striping is seldom used outside
of RDF ontologies.
One feature of RDF that makes it especially attractive is that its semantics
have been formalized using mathematical logic. There are now a number
of ontology languages that extend RDF and that also have formal seman-
tics. The DARPA Agent Markup Language (DAML) is a DARPA project
that produced the DAML+OIL language. This language has recently been
superseded by the Web Ontology Language (OWL). OWL is a standard of
the World Wide Web Consortium (W3C). The RDF and OWL standards are
available on the W3C website (www.w3c.org). Both RDF and OWL will be
discussed in much more detail in the rest of this book.
Summary
- Relationships connect concepts to each other.
- XML has only one explicit kind of relationship: containment.
- Relationships can be specified in XML in two ways:
- adding a new layer (striping),
- using references.