第二章答案1

2

INTRODUCTION TO DATABASE

DESIGN

Exercise 2.1 Explain the following terms briefly: attribute, domain, entity, relation-

ship, entity set, relationship set, one-to-many relationship, many-to-many relationship,

participation constraint, overlap constraint, covering constraint, weak entity set, aggre-

gation,androle indicator.

Entity set - a collection of similar entities such as all of the toys in the toy depart-

Relationship set - a collection of similar relationships

associated with many of another entity. An example of a one-to-many relationship

entity can be associated with many of another entity. An example of a many-

participation constraint says that every department has a manager. A partial

participation constraint says that every employee does not have to be a manager.

Overlap constraint - within an ISA hierarchy, an overlap constraint determines

whether or not two subclasses can contain the same entity.

Covering constraint - within an ISA hierarchy, a covering constraint determines

where the entities in the subclasses collectively include all entities in the superclass.

SalaryEmployee, does every Employee entity necessarily have to be within either

set with a relation Reports-To that relates supervisors and subordinates.

Exercise 2.2 A university database contains information about professors (identified

by social security number, or SSN) and courses (identified by courseid). Professors

teach courses; each of the following situations concerns the Teaches relationship set. For

hold).

1. Professors can teach the same course in several semesters, and each offering must

be recorded.

2. Professors can teach the same course in several semesters, and only the most

Professor

ssn courseId

Professor

ssn courseId

that only the latest offering of a course needs to be recorded.

Exercise 2.3 Consider the following information about a university database: