Databases Reference
In-Depth Information
exemplars of concepts we have. In other words, what we observe depends on
the concepts we use in the observation.
Classification is the operation that associates an object with a concept.
The inverse operation, instantiation, gives an instance of a concept. The set
of objects that are an instance of a concept at some time is called the popula-
tion of that concept at that time.
An entity type is a concept whose instances are individual and identifi-
able objects. Objects that are instances of an entity type are called entities.
Figure 2.2 shows a simple example of entity and entity type.
All entities are instances of some entity type, but an entity may be an
instance of more than one entity type. For example, in Figure 2.2 the entity
shown could also be an instance of Doctor.
If there is a thing in which we are interested, but we are not able to
classify it in any of the concepts we have, then we have to form a new concept
of which that thing could be an instance. In contrast, there may be con-
cepts without instances in the usual domains. The typical example is Uni-
corn. In conceptual modeling, we do not show interest in concepts without
instances.
Some concepts are associative, in the sense that their instances relate
two or more entities. Relationship types are concepts whose instances are rela-
tionships. Figure 2.3 shows an example of relationship type Reads between
Person and Book.
A particular case of relationship is the reference relationship. In princi-
ple, each entity in the domain must have at least one name that allows us to
distinguish among entities. A name is a linguistic object that we use to refer
to an entity. Names are also entities and, therefore, instances of some entity
Concept/
entity type
Person
instance of
entity
Figure 2.2
Entities as instances of concepts.
