Schema Translation - Information Systems Reengineering, Integration and Normalization - page 116

Case 3: Many-to-Many Cardinality

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Fig. 3.47 Schema translation of many-to-many cardinality

Rule 5: Mapping Aggregation from RDB to DTD
An aggregation specifies a whole-part relationship between an aggregate such that
a class represents the whole and a constituent represents part. DTD can construct
part-of relationship in the element content. For example, in Fig. 3.48 , entity B, en-
tity C, and relationship R1 form an aggregate entity that is related to another entity
A. They can be mapped into DTD as follows:

Rule 6: Mapping ISA Relationship from RDB to DTD
The isa defines as relationship between a subclass entity to a superclass entity. In
DTD, we transform each subclass entity as a child element that refers to its parent ele-
ment such that each parent element can have zero to one child elements as (Fig. 3.49 ):

Rule 7: Mapping Generalization from RDB to DTD The generalization defines
a relationship between entities to build a taxonomy of classes: One entity is a more
general description of a set of other entities. In DTD, we transform the general
superclass entity into an element, the element type originating from the superclass.
For example, in Figs. 3.50 and 3.51 , we present the generalization of entity B and
entity C into entity A in DTD.