Referential Integrity for Database Design - High-Performance Web Databases: Design, Development, and Deployment - page 427

Exhibit 28-3. Proposed notation for attribute referential integrity.

(r2), where is a projection operation in the relational algebra. For an at-

tribute referential integrity constraint to make sense, either

∏

α = β or α and

β must be compatible sets of attributes.

The E-R Model

Exhibit 3 shows a proposed notation for attribute referential integrity on
the E-R diagram. RI in the small rectangle indicates that the participating
entity types E1 and E2 contain the attributes whose referential integrity
should be maintained.

On the line between the RI rectangle and the participating entity types,
the referential relationship is represented (i.e., the referencing or refer-
enced attribute is specified), where an underlined attribute represents a
referenced attribute. As defined, the referenced attribute contains a super-
set of all values appearing in the referencing attribute.

Exhibit 4 shows an example of a new E-R model for the previous exam-
ple. Unlike the first example, there is no need to create a new entity type
solely for representing the referential integrity constraint. On the entity
types

_ and , the vehicle type attribute ( VehTypeName )

RENTAL PACKAGE VEHICLE

Exhibit 28-4. An example of the extended E-R model.