Database Reference
In-Depth Information
come across is
owl:allValuesFrom
. This value constraint is much more restric-
tive than “some” as it creates an anonymous class that contains
only
things that are
sold in a Licensed Establishment
and nothing else.
Essentially, this says that beer
can only be sold somewhere that has a license to sell alcohol (a licensed establish-
ment); it cannot be sold anywhere else. So, you can see how careful we have to be
with applying this restriction as in many cases it would convey incorrect knowledge.
For example, applying the universal quantifier (only) to our first example would make
the statement “Every Pub sells only Beer or nothing” and would mean that there are
not any Pubs that sold other beverages, which would be a tragedy for cider drinkers.
The other important point to note about “only” is that the anonymous class could
be empty (whereas for Some we know it must contain at least one member). This
means that what we are really saying in the second example is that “Beer can only
be sold in Licensed Establishments and nowhere else, but it might not be sold at all”
(a tragedy for beer drinkers).
An ontology pattern that is often seen is when both types of restriction are used.
This changes the statement to “Beer is sold in Licensed Establishments only, and
nowhere else,
and it has to be sold.
” The existential quantifier “some” adds the itali-
cized part of the statement. This is known as a “closure axiom”:
Every Beer is only sold in
Licensed Establishments.
Class: Beer
SubClassOf: owl:Thing that isSoldIn
onlysome LicensedEstablishment
The Rabbit statement is simpler in that it assumes that “only” implies the closure axiom
as this is the most likely meaning in natural language. In the OWL file, you may notice
that your ontology editor separates the
onlysome
keyword out into two statements:
Class: Beer
SubClassOf: isSoldIn only LicensedEstablishment and isSoldIn some
LicensedEstablishment
3
Another value constraint is
owl:hasValue
, which corresponds directly to the
CNL term “has value” and the Manchester Syntax keyword
value
. It is used to
specify a particular value that a property must take when used with a certain subject
class or individual. As we saw previously:
The Isis Tavern has name
“The Isis Tavern”.
9.3.5 c
arDinaLity
c
Onstraints
As well as value constraints restricting properties, OWL specifies cardinality con-
straints: the maximum, minimum, or exact number of values that a property can take
when applied to a particular concept. These are often known as Qualified Cardinality
Restrictions. (The designation “qualified” means that the property restriction only
