Generalization

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

• 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.