THE RELATIONAL DATABASE MODEL: INTRODUCTION - Fundamentals of Database Management Systems - page 109

Primary and Candidate Keys
Primary Keys Figure 5.2 contains two relations, the SALESPERSON relation
and the CUSTOMER relation, from General Hardware Company's relational
database. The SALESPERSON relation has four rows, each representing one
salesperson. Also, the SALESPERSON relation has four columns, each representing
a characteristic of salespersons. Similarly, the CUSTOMER relation has nine rows,
each representing a customer, and four columns.
A relation always has a unique primary key . A primary key (sometimes
shortened in practice to just ''the key'') is an attribute or group of attributes whose
values are unique throughout all rows of the relation. In fact, the primary key
represents the characteristic of a collection of entities that uniquely identifies each
one. For example, in the situation described by the relations in Figure 5.2, each
salesperson has been assigned a unique salesperson number and each customer
has been assigned a unique customer number. Therefore the Salesperson Number
attribute is the primary key of the SALESPERSON relation and the Customer
Number attribute is the primary key of the CUSTOMER relation. As in Figure 5.2,
we will start marking the primary key attribute(s) with a single, solid underline.
The number of attributes involved in the primary key is always the minimum
number of attributes that provide the uniqueness quality. For example, in the
SALESPERSON relation, it would make no sense to have the combination of
Salesperson Number and Salesperson Name as the primary key because Salesperson
Number is unique by itself. However, consider the situation of a SALESPERSON

(a) SALESPERSON relation

Salesperson Salesperson Commission Year

Number Name Percentage of Hire

137 Baker 10 1995

186 Adams 15 2001

204 Dickens 10 1998

361 Carlyle 20 2001

(b) CUSTOMER relation

Customer Customer Salesperson

Number Name Number HQ City

0121 Main St. Hardware 137 New York

0839 Jane's Stores 186 Chicago

0933 ABC Home Stores 137 Los Angeles

1047 Acme Hardware Store 137 Los Angeles

1525 Fred's Tool Stores 361 Atlanta

1700 XYZ Stores 361 Washington

1826 City Hardware 137 New York

FIGURE5.2
General Hardware Company
relational database

2198 Western Hardware 204 New York

2267 Central Stores 186 New York