Page 129 - DCAP402_DCAO204_DATABASE MANAGEMENT SYSTEM_MANAGING DATABASE

Database Management Systems/Managing Database

Notes
Figure 7.7: Class Hierarchy

name
ssn lot

Employees

ISA
name
hours_worked
hourly_wages

Hourly_Emps Contract_Emps

1. We can map each of the entity sets Employees, Hourly_Emps, and Contract_Emps to a
distinct relation. The relation for Hourly_Emps includes the hourly_wages and
hours_worked attributes of Hourly_Emps. It also contains the key attributes of the
superclass, which serve as the primary key for Hourly_Emps, as well as a foreign key
referencing the superclass (Employees). For each Hourly_Emps entity, the value of the
name and lot attributes are stored in the corresponding row of the superclass (Employees).
2. We can create just two relations, corresponding to Hourly_Emps and Contract_Emps. The
relation for Hourly_Emps includes all the attributes of Hourly_Emps as well as all the
attributes of Employees (i.e., ssn, name, lot, hourly_wages, hours_worked).
The first approach is general and is always applicable. Queries in which we want to examine all
employees and do not care about the attributes specific to the subclasses are handled easily
using the Employees relation.
The second approach is not applicable if we have employees who are neither hourly employees
nor contract employees, since there is no ways to store such employees. Also, if an employee is
both an Hourly_Emps and a Contract_Emps entity, then the name and lot values are stored twice.
This duplication can lead to some of the anomalies.
Translating ER Diagrams with Aggregation: Translating aggregation into the relational model
is easy because there is not real distinction between entities and relationships in the relational
model.
Consider the ER diagram shown in Figure 7.8. The Employees, Project, and Departments entity
sets and the Sponsors relationship set are mapped as described in previous sections. For the
Monitors relationship set, we create a relation with the following attributes: the key attributes
of Employees (ssn), the key attributes of Sponsors (did, pid), and the descriptive attributes of
Monitors (until).

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