Schema Translation - Information Systems Reengineering, Integration and Normalization - page 99

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Fig. 3.24 Mapped EER model for university enrollment

• Substep2isnotappliedbecauseoflackofmultiple(alternative)accesspaths
between two segments in a hierarchical database.
• Substep4,Derivationofentitiesofanm:nbinaryrelationship, is not applied.

To implement an m:n relationship in a hierarchical schema, in general, two defini-
tion trees are needed, with each segment type represented as a parent segment in one
tree, but as a child segment in another as shown below in Fig. 3.25
However, some hierarchical DBMSs provide mechanisms whereby it is possible
to define logical views on one physical storage structure. The IMS database sup-
ports m:n relationships through pair logical segments as shown in Fig. 3.26 .
Substeps 4, 5, and 6 are not applied because of lack of similar data structure in
the hierarchical schema.