Metadata, Resources, and the Resource Description Framework
Since the XML description is just another way of expressing a graph, it is possi-
ble to do exactly the same operations on the XML description as on the graph
drawn on paper. It is somewhat complicated to go back to the graph from the
XML representation, however, because the same XML representation can be
drawn as several different graphs (all graphs which have the same format are
the same to a mathematician).
Because you are working with triples of information that contain relationships
to each other, you can also filter out information with a much higher degree of
precision than alternate technologies. If you have an attribute-value-table, you
can filter out information based on the value of the attributes. If another site
does not share the same attributes and values, you cannot use information
from that site in your filtering process. However, if both your site and the other
site express the information about yourself in a format that relates the state-
ments to you, you will be able to filter out information to create a much more
precise selection of information for the user. This can, for instance, be applied
to a site which creates a home page from two different sites (what might be
termed a personal portal) based on the user s preferences. It can be used to
describe the copyright restrictions you have set on the use of the information,
and it can be used to describe the relationship of your site with the site provid-
ing the personal portal.
As you can understand, whether you call the arcs Evidence A or a horsetail
does not matter to the underlying graph. However, the properties of the ele-
ments can change the graph. You need to declare what elements you are going
to use, and which properties they are going to have. These properties will
change the way you can make statements. They will change how the terms
interrelate, both in the current graph and with elements in other graphs. You
need to declare the elements and their properties, and the structure in which
they can occur, the schema of the graph.
Schema in the present context refers to a data structure. It has gained that use
in the database industry, where it describes the structure of a database table:
the columns and their headings, and what restrictions there may be on the data
in the columns.
Both the properties and the RDF elements are objects in their own right, and
can be the subjects of statements in themselves. This means that there can be
graphs chained to the graph you look at orthogonally; or in any number of
dimensions, because to a mathematician, the paper drawing is not necessarily
the clearest description.
A schema does not define only the data structure for your graph. It also defines
the terms you can use in it, the vocabulary of the description. For instance, the
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