Databases Reference
In-Depth Information
13.3.1 The Choice of the Conceptual Model
The purpose of a conceptual schema is to describe in a formal way the part of
the real world to represent into a DB. The choice of the conceptual language
influences the modeling tasks and determines the necessary knowledge to
perform those tasks. There is a general agreement, although never standard-
ized, to use an E/R model [7] or one of its extensions as a high-level formal-
ism to describe conceptual DB schemas. The extended E/R model used in
this chapter is summarized by the metamodel in Figure 13.4.
In this model, entities represent concrete or abstract objects relevant to
the given real world. They are described by lists of attributes that may be sim-
ple or composite, monovalued or multivalued. Relationships are binary or
n-ary associations between not necessarily distinct entities. Each link between
an entity and a relationship materializes the role played by the entity in the
relationship. Each role is characterized by cardinalities that specify, on one
hand, the number of entity instances involved in a relationship instance, and
on the other hand, the number of relationship instances in which the same
entity instance participates. Each of these numbers is actually represented
by a couple of values, minimal cardinality and maximal cardinality, which
respectively specify the minimum and maximum instances involved in each
role. Relationships may or may not have their own attributes. Entity
instances are identified by one or several of their attributes. Relationship
instances are identified by a combination of identifiers of the participating
Conceptual
schema
1-N
0-N
Relationship
type
Entity
type
0-N
2-N
Role
GEN
AGG
0-N
1-N
Attribute
type
1-1
1-1
0-N
Has-a
Has-a
0-N
0-N
Has-a
Has-a
Has-a
0-1
Constraint
type
0-1
0-N
Figure 13.4
The E/R metamodel.
