Database Reference
In-Depth Information
applies when the property value is a particular concept, rather than applying to the
property globally.) For example, we can describe a semidetached house as a kind of
house that is attached to exactly one other house.
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Every Semi Detached House is
attached to exactly 1 Semi
Detached House.
Class: SemiDetachedHouse
SubClassof: House that
(isAttatchedTo exactly 1
SemiDetachedHouse
Similarly, “max” or “min” (in Rabbit “at most,” “at least”) can be used to denote
the maximum or minimum value that the property can take.
9.3.6 i
ntersectiOn
anD
u
niOn
As we know, OWL is set based, and two commonly used concepts are Union (where
we are talking about any individuals that belong to either one or both overlapping or
possibly nonoverlapping sets) and Intersection (where we are interested in only those
individuals that are members of both sets at the same time). In the OWL/XML syn-
tax, these are known as
owl:UnionOf
and
owl:IntersectionOf
, respectively.
These correspond to the logical OR and AND, but they do not quite correspond to
how we understand the words
or
and
and
in straightforward English. Rabbit enables
the reader to clearly differentiate between an Exclusive OR, “you may have tea or
coffee” (but not both), and the Inclusive OR, “A farm can have land that comprises
arable fields and/or pasture and/or meadow,” meaning that it can have any combina-
tion. This last form is always preceded by “one or more of” in Rabbit. So, the Rabbit
and Manchester versions of the last statement would be
Every Farm has land one or more
of Arable Fields or Pasture or
Meadow.
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Class: Farm
subClassOf: hasLand some
ArableField or Pasture or Meadow
Figure 9.2
visualizes this statement in terms of sets. Note that “or” means “one or
the other or both” (so the farm in the example contains some land that is arable land,
pasture land, or meadow, or land that is a mixture of two or three of them). As we
can see from the members of the sets in the Venn diagram of Figure 9.2, there are a
number of farmland instances that lie within the union of the three sets. Conversely,
however, an intersection restriction would mean that every option mentioned must
be true. So, if we had said
Class: Farm
subClassOf: contains some ArableLand and PastureLand and Meadow
we would be stating that the land was Arable and Pasture and Meadow all at the same
time, that is, referring to the center of the Venn diagram where all three circles inter-
sect, which is empty. This example illustrates the beginner's mistake of using the word
and
in English to refer to a logical OR. In fact, Rabbit does not allow the use of “and”
in order to prevent this very common mistake, so intersection must be stated through
