extended ER features

The basic E-R concepts can model most database features, some aspects of a database may be more aptly expressed by certain extensions to the basic E-R model. The extended E-R features are specialization, generalization, higher- and lower-level entity sets, attribute inheritance, and aggregation.

1. Specialization

An entity set may include subgroupings of entities that are distinct in some way from other entities in the set. For instance, a subset of entities within an entity set may have attributes that are not shared by all the entities in the entity set. The E-R model provides a means for representing these distinctive entity groupings.

Consider an entity set person, with attributes name, street, and city. A person may be further classified as one of the following:

• customer

• employee

Each of these person types is described by a set of attributes that includes all the attributes of entity set person plus possibly additional attributes. For example, customer entities may be described further by the attribute customer-id, whereas employee entities may be described further by the attributes employee-id and salary. The process of designating subgroupings within an entity set is called specialization. The specialization of person allows us to distinguish among persons according to whether they are employees or customers.

2.Generalization

The refinement from an initial entity set into successive levels of entity subgroupings represents a top-down design process in which distinctions are made explicit. The design process may also proceed in a bottom-up manner, in which multiple entity sets are synthesized into a higher-level entity set on the basis of common features. The database designer may have first identified a customer entity set with the attributes name, street, city, and customer-id, and an employee entity set with the attributes name, street, city, employee-id, and salary.

There are similarities between the customer entity set and the employee entity set in the sense that they have several attributes in common. This commonality can be expressed by generalization, which is a containment relationship that exists between a higher-level entity set and one or more lower-level entity sets. In our example, person is the higher-level entity set and customer and employee are lower-level entity sets. Higher- and lower-level entity sets also may be designated by the terms superclass and subclass, respectively. The person entity set is the superclass of the customer and employee subclasses.

For all practical purposes, generalization is a simple inversion of specialization.            

3.Attribute Inheritance

A crucial property of the higher- and lower-level entities created by specialization and generalization is attribute inheritance. The attributes of the higher-level entity sets are said to be inherited by the lower-level entity sets. For example, customer and employee inherit the attributes of person. Thus, customer is described by its name, street, and city attributes, and additionally a customer-id attribute; employee is described by its name, street, and city attributes, and additionally employee-id and salary attributes.

A lower-level entity set (or subclass) also inherits participation in the relationship sets in which its higher-level entity (or superclass) participates. The officer, teller, and secretary entity sets can participate in the works-for relationship set, since the superclass employee participates in the works-for relationship. Attribute inheritance applies through all tiers of lower-level entity sets. The above entity sets can participate in any relationships in which the person entity set participates. Whether a given portion of an E-R model was arrived at by specialization or generalization,the outcome is basically the same:

• A higher-level entity set with attributes and relationships that apply to all of its lower-level entity sets

• Lower-level entity sets with distinctive features that