Data Modeling

This assignment is ungraded and should not be submitted. The questions review key concepts from the reading for this module.

Expected duration: 45 minutes

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Assignment

Answer the following questions from Fundamentals of Database Management Systems (Gillenson, 2011).

1. Explain and compare the cardinality of a relationship and the modality of a relationship. (Question 2.5)

   The cardinality of a relationship describes the maximum number of entities that can participate in it—e.g., one-to-one, one-to-many, and many-to-many relationships. The modality of a relationship is the minimum number of entities that can participate in it. The modality is often expressed as “may” vs. “must” participation—e.g., a student may have declared a major, but a graduate must have one.

2. What is an associative entity? How does intersection data relate to an associative entity? (Question 2.10)

   An associative entity is one that arises from a many-to-many relationships. Intersection data describes the relationship between the two entities and comprises the associative entity’s attributes.

3. What is a dependent (or weak) entity? (Question 2.16)

   A dependent entity is one that exists only as long as another entity (on which it depends) exists. A dependent entity is unique only within the context of the other entity.

4. Draw an entity-relationship diagram that describes the following business environment. (Exercise 2.1)

   The city of Chicago, IL, wants to maintain information about its extensive system of high schools, including its teachers and their university degrees, its students, administrators, and the subjects that it teaches.

   Each school has a unique name, plus an address, telephone number, year built, and size in square feet. Students have a student number, name, home address, home telephone number, current grade, and age. Regarding a student’s school assignment, the school system is only interested in keeping track of which school a student currently attends. Each school has several administrators, such as the principal and assistant principals. Administrators are identified by an employee number and also have a name, telephone number, and office number.

   Teachers are also identified by an employee number and each has a name, age, subject specialty such as English (assume only one per teacher), and the year that they entered the school system. Teachers tend to move periodically from school to school and the school system wants to keep track of the history of which schools the teachers has taught in, including the current school. Included will be the year in which the teacher entered the school, and the highest pay rate that the teacher attained at the school. The school system wants to keep track of the universities that each teacher attended, including the degrees earned and the years in which they were earned. The school system wants to record each university’s name, address, year founded, and Internet URL (address). Some teachers, as department heads, supervise other teachers. The school system wants to keep track of these supervisory relationships but only for teachers’ current supervisors.

   The school system also wants to keep track of the subjects that it offers (e.g., French I, Algebra III, etc.). Each subject has a unique subject number, a subject name, the grade level in which it is normally taught, and the year in which it was introduced in the school system. The school system wants to keep track of which teacher taught which student which subject, including the year this happened and the grade received.