Entity-relationship Model and E-r Diagram
Exercises
9
address driver-id person name owns license
model year car report-number location date
driver
participated
accident
damage-amount
Figure 2.1
E-R diagram for a Car-insurance company.
Exercises
2.1 Explain the distinctions among the terms primary key, candidate key, and superkey. Answer: A superkey is a set of one or more attributes that, taken collectively, allows us to identify uniquely an entity in the entity set. A superkey may contain extraneous attributes. If K is a superkey, then so is any superset of K. A superkey for which no proper subset is also a superkey is called a candidate key. It is possible that several distinct sets of attributes could serve as candidate keys. The primary key is one of the candidate keys that is chosen by the database designer as the principal means of identifying entities within an entity set. 2.2 Construct an E-R diagram for a car-insurance company whose customers own one or more cars each. Each car has associated with it zero to any number of recorded accidents. Answer: See Figure 2.1 2.3 Construct an E-R diagram for a hospital with a set of patients and a set of medical doctors. Associate with each patient a log of the various tests and examinations conducted. Answer: See Figure 2.2 2.4 A university registrar’s office maintains data about the following entities: (a) courses, including number, title, credits, syllabus, and prerequisites; (b) course offerings, including course number, year, semester, section number, instructor(s), timings, and classroom; (c) students, including student-id, name, and program; and (d) instructors, including identification number, name, department, and title. Further, the enrollment of students in courses and grades awarded to students in each course they are enrolled for must be appropriately modeled. Construct an E-R diagram for the registrar’s office. Document all assumptions that you make about the mapping constraints. Answer: See Figure 2.3. In the answer
9
address driver-id person name owns license
model year car report-number location date
driver
participated
accident
damage-amount
Figure 2.1
E-R diagram for a Car-insurance company.
Exercises
2.1 Explain the distinctions among the terms primary key, candidate key, and superkey. Answer: A superkey is a set of one or more attributes that, taken collectively, allows us to identify uniquely an entity in the entity set. A superkey may contain extraneous attributes. If K is a superkey, then so is any superset of K. A superkey for which no proper subset is also a superkey is called a candidate key. It is possible that several distinct sets of attributes could serve as candidate keys. The primary key is one of the candidate keys that is chosen by the database designer as the principal means of identifying entities within an entity set. 2.2 Construct an E-R diagram for a car-insurance company whose customers own one or more cars each. Each car has associated with it zero to any number of recorded accidents. Answer: See Figure 2.1 2.3 Construct an E-R diagram for a hospital with a set of patients and a set of medical doctors. Associate with each patient a log of the various tests and examinations conducted. Answer: See Figure 2.2 2.4 A university registrar’s office maintains data about the following entities: (a) courses, including number, title, credits, syllabus, and prerequisites; (b) course offerings, including course number, year, semester, section number, instructor(s), timings, and classroom; (c) students, including student-id, name, and program; and (d) instructors, including identification number, name, department, and title. Further, the enrollment of students in courses and grades awarded to students in each course they are enrolled for must be appropriately modeled. Construct an E-R diagram for the registrar’s office. Document all assumptions that you make about the mapping constraints. Answer: See Figure 2.3. In the answer