Database Management

and manipulation. It is normally easier for managers to understand the relational model

(see Figure 3.5) than other database models.

Databases based on the relational model include IBM DB2, Oracle, Sybase, Microsoft

SQL Server, Microsoft Access, and MySQL. Oracle is currently the market leader in generalpurpose

databases, with over 40 percent of the $16.5 billion database market. IBM comes

in second with about 21 percent, and Microsoft third with about 19 percent.9

In the relational model, each row (or record) of a table represents a data entity, with the

columns (or fields) of the table representing attributes. Each attribute can accept only certain

values. The allowable values for these attributes are called the domain. The domain for a

particular attribute indicates what values can be placed in each column of the relational table.

For instance, the domain for an attribute such as gender would be limited to male or female.

A domain for pay rate would not include negative numbers. In this way, defining a domain

can increase data accuracy.

Manipulating Data

After entering data into a relational database, users can make inquiries and analyze the data.

Basic data manipulations include selecting, projecting, and joining. Selecting involves eliminating

rows according to certain criteria. Suppose a project table contains the project

number, description, and department number for all projects a company is performing. The

president of the company might want to find the department number for Project 226, a sales

manual project. Using selection, the president can eliminate all rows but the one for Project

226 and see that the department number for the department completing the sales manual

project is 598.

relational model

A database model that describes

data in which all data elements are

placed in two-dimensional tables,

called relations, which are the

logical equivalent of files.

domain

The allowable values for data

attributes.

selecting...