The Relational Model and Normalization - Database Processing: Fundamentals, Design, and Implementation - page 122

EmployeeNumber FirstName LastName Department Email Phone

100 Jerry Johnson Accounting [e-mail] 834-1101

200 Mary Abernathy Finance [e-mail] 834-2101

300 Liz Smathers Finance [e-mail] 834-2102

400 Tom Caruthers Accounting [e-mail] 834-1102

500 Tom Jackson Production [e-mail] 834-4101

600 Eleanore Caldera Legal [e-mail] 834-3101

834-3102

700 Richard Bandalone Legal [e-mail]

Further, in a relation, all of the values in a column are of the same kind. If, for example, the
second column of the first row of a relation has FirstName, then the second column of every row
in the relation has FirstName. This is an important requirement that is known as the domain in-
tegrity constraint , where the term domain means a grouping of data that meets a specific type
definition. For example, FirstName would have a domain of names such as Albert , Bruce , Cathy ,
David , Edith , and so forth, and all values of FirstName must come from the names in that domain.
The EMPLOYEE table shown in Figure 3-5 meets these criteria and is a relation.

Figure 3-5
Sample EMPLOYEE
relation

By The WAy Columns in different relations may have the same name. In Chapter 2, for
example, two relations had a column named SKU. When there is risk of
confusion, we precede the column name with the relation name followed by a period.
Thus, the name of the SKU column in the SKU_DATA relation is SKU_DATA.SKU, and
column C1 of relation R1 is named R1.C1. Because relation names are unique within a
database and because column names are unique within a relation, the combination of
relation name and column name uniquely identifies every column in the database.

Figure 3-6
Nonrelational table—
Multiple Entries
per Cell

Each cell of a relation has only a single value or item; multiple entries are not allowed.
The table in Figure 3-6 is not a relation because the Phone values of employees Caruthers and
Bandalone store multiple phone numbers.

EmployeeNumber FirstName LastName Department Email Phone

100 Jerry Johnson Accounting [e-mail] 834-1101

200 Mary Abernathy Finance [e-mail] 834-2101

300 Liz Smathers Finance [e-mail] 834-2102

400 Tom Caruthers Accounting [e-mail] 834-1102,

834-1191,
834-1192

500 Tom Jackson Production [e-mail] 834-4101

600 Eleanore Caldera Legal [e-mail] 834-3101

834-3102,
834-3191

700 Richard Bandalone Legal [e-mail]