Page 183 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Database Management Systems/Managing Database

Notes

Task “Specializing transformations change the extension of a process while
preserving the frame of reference”. Explain

Rules

The primary rule of generalization hierarchies is that each instance of the supertype entity must
appear in at least one subtype; likewise, an instance of the subtype must appear in the supertype.
Subtypes can be a part of only one generalization hierarchy. That is, a subtype can not be related
to more than one supertype. However, generalization hierarchies may be nested by having the
subtype of one hierarchy be the supertype for another.

Subtypes may be the parent entity in a relationship but not the child. If this were allowed, the
subtype would inherit two primary keys.

10.7 Summary

 The objective of data modeling is to design a data structure for a database fitting as good
as possible with some relevant world, often related to an organization with some
information need.

 In general there is some relationship between a data model and a part of the existing
world, but it is also possible that a data model has a relationship with some imaginary and
abstract world.
10.8 Keywords

Datalog Program: Datalog program as either output relations or input relations.
Data modeling: Data modeling, using a specific data model type, and as a unique activity during
information system design.
Specialization: specialization as a restriction on the extension of a process.

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