Information Technology Reference
In-Depth Information
Figure 5.6
Manager
A Simplified ER Diagram
Showing the Relationship
Between the Manager,
Department, and Project Tables
Supervises
Department
Performs
Project
Data Table 1: Project Table
Project number
155
Figure 5.7
Description
Payroll
Dept. number
257
Linking Data Tables to Answer
an Inquiry
In finding the name and hire date of
the manager working on the sales
manual project, the president needs
three tables: project, department,
and manager. The project
description (Sales manual) leads to
the department number (598) in the
project table, which leads to the
manager's SSN (098-40-1370) in the
department table, which leads to the
manager's name (Fiske) and hire
date (01-05-1985) in the manager
table. Again, note that some
organizations might use employee
number instead of Social Security
number (SSN).
498
Widgets
632
226
Sales manual
598
Data Table 2: Department Table
Dept. number
257
Dept. name
Accounting
Manager SSN
005-10-6321
632
Manufacturing
549-77-1001
598
Marketing
098-40-1370
Data Table 3: Manager Table
SSN
005-10-6321
Last name
Johns
First name
Francine
Hire date
10-07-1997
Dept. number
257
549-77-1001
Buckley
Bill
02-17-1979
632
598
098-40-1370
Fiske
Steven
01-05-1985
One of the primary advantages of a relational database is that it allows tables to be linked,
as shown in Figure 5.7. This linkage is especially useful when information is needed from
multiple tables. For example, the manager's Social Security number is maintained in the
manager table. If the Social Security number is needed, it can be obtained by linking to the
manager table.
The relational database model is by far the most widely used. It is easier to control, more
flexible, and more intuitive than other approaches because it organizes data in tables. As
shown in Figure 5.8, a relational database management system, such as Access, provides tips
and tools for building and using database tables. In this figure, the database displays infor-
mation about data types and indicates that additional help is available. The ability to link
relational tables also allows users to relate data in new ways without having to redefine com-
plex relationships. Because of the advantages of the relational model, many companies use it
for large corporate databases, such as those for marketing and accounting. The relational
model can also be used with personal computers and mainframe systems. A travel reservation
company, for example, can develop a fare-pricing system by using relational database tech-
nology that can handle millions of daily queries from online travel companies, such as
Expedia, Travelocity, and Orbitz.
