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in the relational schema, that is, a link represented by a foreign key
from the referencing entity (the one whose role cardinality is 1-1) to
the other. Possible attributes of the relationship are transferred to
the referencing entity.
R6: Any conceptual relationship can be transformed into a relation
in the logical schema. This relation references all the participating
entities.
·
Generalization hierarchies are not supported by the relational model. There
are different ways to represent them, specified in the following three rules:
R7: Given a generalization hierarchy of conceptual entities, each
entity of this hierarchy is a candidate to become a base relation in
the logical level. Generalization links between these conceptual enti-
ties are represented by references from specialized entities to the
generic entity.
·
R8: Given a hierarchy of conceptual entities, only specialized entities
are candidates to become base relations. The generic entity is
defined as a view computed by the union of the previous relations.
This rule assumes that the generalization hierarchy is complete, that
is, each generic instance is also an instance of one of the specialized
entities.
·
R9: Given a hierarchy of conceptual entities, only the generic entity
is a candidate to be a base relation. Specialized entities are repre-
sented as views, computed by a selection on the base relation. This
selection assumes that a new attribute, generically named role, is
defined in the base relation such that its domain is composed of the
names of the specialized entities.
·
Figure 13.17 illustrates a successive application of those three rules to the
same generalization hierarchy.
13.4.3.2 Mapping a Conceptual Object-Oriented Schema Into a
Relational Schema
The difference between an extended E/R model, such as the one defined
in Figure 13.4, and an object-oriented model, as defined in OMT [35] and
UML [2], is mainly the addition of operations on entities and relationships,
and
messages
between
objects.
Consequently,
the
structural
mapping
