RELATIONAL DATA MODEL FUNDAMENTALS - Database Design and Development: An Essential Guide for IT Professionals - page 238

EMPLOYEE Relation

Columns

SocSecNumber EmployeeName Phone Position DeptCode

214-56-7835 Robert Moses 516-777-9584 Programmer 501

123-44-5546 Kassia Raj 718-312-4488 Analyst

101-54-3838 Andrew Rogers 212-313-1267 Manager 408

213-36-7854 Samuel Prabhu 212-126-3428 Controller 201

311-33-4520 Kaitlin Jones 718-567-4321 Assistant

512-22-8542 Carey Smith 732-346-5533 Senior VP 301

111-22-3344 Amanda Lupo 908-212-5629 Executive VP 101

122-65-5378 215-576-4598 DBA 501

Tabitha Williams

Figure 8-3 Employee relation: tuples.

Figure 8-3 shows the rows or tuples for the EMPLOYEE relation.
If there are 5000 employees in the organization, the relation will contain 5000
rows. The number of tuples in a relation is known as the cardinality of the relation.
For an EMPLOYEE relation with 5000 rows, the cardinality is 5000.
Now, because a relation is considered as a mathematical set, this EMPLOYEE
relation is a set of 5000 data elements. Manipulation of data in the EMPLOYEE
relation, therefore, becomes a set operation. Later on, you will study set operations
and learn how to apply these in working with data in a relational model.
Each row represents a particular employee. Look at the row for the employee
Carey Smith. Note the value shown under each column in this row. Each of the
values in the columns describes the employee Carey Smith. Each value represents
one piece of data about the employee. All data for the employee are contained in
the specific row.

Primary Key

As mentioned above, in a relation, each tuple represents one instance of the rela-
tion. In an EMPLOYEE relation with 5000 rows, each row represents a particular
employee. But, how can we know which row represents an employee we are looking
for? To identify a row uniquely, we can use the attribute values. We may say that if
the value of the attribute EmployeeName is “Carey Smith” that row represents this
particular employee. What if there is another Carey Smith in the organization? You
need some attribute whose values will uniquely identify individual tuples. Note that
the attribute SocSecNumber can be used to identify a tuple uniquely.
Given below are definitions of identifiers in a relation:

Superkey. A set of attributes that uniquely identifies each tuple in a relation.

Key. A minimal set of attributes that uniquely identifies each tuple in a relation.

Composite key. A key consisting of more than one attribute.

Candidate key. A set of attributes that can be chosen to serve the key.