DEVELOPING AN ER DIAGRAM
The process of database design is an iterative rather than a linear or
sequential process. The verb iterates means “to do again or
repeatedly.” An iterative process is, thus, one based on repetition of
processes and procedures.
Building an ERD usually involves the following activities:
_ Create a detailed narrative of the organization’s description of
operations.
_ Identify the business rules based on the description of operations.
_ Identify the main entities and relationships from the business rules.
_ Develop the initial ERD.
_ Identify the attributes and primary keys that adequately describe the
entities.
_ Revise and review the ERD.
To illustrate the use of the iterative process that ultimately yields a
workable ERD, let’s start with an initial interview with the Tiny College
administrators. The interview process yields the following business
rules:
1. Tiny College (TC) is divided into several schools: a school of
business, a school of arts and sciences, a school of education,
and a school of applied sciences. Each school is administered by
a dean who is a professor. Each dean can administer only one
school. Therefore, a 1:1 relationship exists between PROFESSOR
and SCHOOL. Note that the cardinality can be expressed by (1,1)
for the entity PROFESSOR and by (1,1) for the entity SCHOOL.
(The smallest number of deans per school is one, as is the
largest number, and each dean is assigned to only one school.)
2. Each school is composed of several departments. For example,
the school of business has an accounting department, a
management/marketing department, an economics/finance
department, and a computer information systems department.
Note again the cardinality rules: the smallest number of
departments operated by a school is one, and the largest
number of departments is indeterminate (N). On the other hand,
each department belongs to only a single school; thus, the
cardinality is expressed by (1,1). That is, the minimum number of
schools that a department belongs to is one, as is the maximum
number.
3. Each department may offer courses. For example, the
management/marketing department offers courses such as
Introduction to Management, Principles of Marketing, and
Production Management. Note that this relationship is based on
the way Tiny College operates. If, for example, Tiny College had
some departments that were classified as “research only,” those
