Database Reference
In-Depth Information
To state that the individual is the same individual as another, OWL has the key-
word
owl:sameAs
. In reasoning terms, this means that
all
information about one
individual also applies to the other individual.
The Frog and Frigate and The Frog
and Frigate [Big Breweries] are
the same thing.
We have already seen how Linked Data frequently uses the
owl:sameAs
property
to marry different datasets, but caution is needed as it only applies when the information
on both sides is equally valid for both individuals. If not, it is better to choose another
relationship to link the two, such as “related to,” or a property that describes a more
specific semantic relationship as appropriate: “belongs to,” “owns,” “part of,” and so on.
9.3.4 v
aLue
c
Onstraints
Value constraints are axioms that restrict the values that can be assigned to a class.
An example of the simplest form of value constraint is
Every Pub sells Beer.
Class:Pub
SubClassOf: owl:Thing
that sells some Beer
This states that a pub must sell some beer, that is, at least one individual from
the set of Beers. In Manchester Syntax, the word
some
is explicitly used, whereas in
Rabbit it is implied. The RDF/XML version of this term, which you may also come
across, is
owl:someValuesFrom
. The use of “some” (whether explicit or implied)
is known as an existential quantifier. This comes from the logic symbol ∃, which
should be read as “there exists,” so the statement we are making is that for every Pub
there exists at least one Beer that is sold there.
It is important to understand exactly what this statement is doing as it helps to
reinforce the way of thinking that is embodied in OWL. OWL and RDFS are set
based: Classes are effectively sets of things. So, the class Pub is a subclass (or subset)
of an anonymous class known as a Restriction, as it is defined by adding a restriction
on the property “sells” so that it is restricted to the range of at least one Beer. The
anonymous class only requires that at least Beer is sold. Other drinks could also be
sold, which is what we want from a Pub. A restriction on the range of a property is
called a “value constraint” as it restricts which values the property can take.
Things get a little more complicated with this second example:
Every Beer is sold only in a
Licensed Establishment or
nothing.
Class:Beer
Subclassof: owl:Thing that isSoldIn
only LicensedEstablishment
The second kind of value constraint uses “only” to represent the ∀ logic symbol,
also known as the universal quantifier. The RDF/XML term for this that you may
