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Consider extending the entity set account by classifying accounts
as being either savings-account or chequing-account.
Each of these is described by the attributes of account plus
additional attributes. (savings has interest-rate and
chequing has overdraft-amount.)
We can express the similarities between the entity sets by
This is the process of forming containment relationships between a
higher-level entity set and one or more lower-level entity sets.
In E-R diagrams, generalization is shown by a triangle, as shown in Figure
Figure 2.19: Generalization
- Generalization hides differences and emphasizes similarities.
- Distinction made through attribute inheritance.
- Attributes of higher-level entity are inherited by lower-level entities.
- Two methods for conversion to a table form:
- Create a table for the high-level entity, plus tables for the
lower-level entities containing also their specific attributes.
- Create only tables for the lower-level entities.
Page created and maintained by Osmar R. Zaï ane
Sun Sep 10 17:02:35 PDT 1995