next up previous
Next: Aggregation Up: The Entity-Relationship Model Previous: Representation of Relationship


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 generalization. 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 2.19.

Figure 2.19: Generalization

Page created and maintained by Osmar R. Zaï ane
Last Update: Sun Sep 10 17:02:35 PDT 1995