Page 1426 - (ISC)² CISSP Certified Information Systems Security Professional Official Study Guide

FIGURE 20.8 Customers table from a relational database

In this example, the table has a cardinality of 3 (corresponding to the

three rows in the table) and a degree of 8 (corresponding to the eight
columns). It’s common for the cardinality of a table to change during
the course of normal business, such as when a sales rep adds new
customers. The degree of a table normally does not change frequently
and usually requires database administrator intervention.

To remember the concept of cardinality, think of a deck of

cards on a desk, with each card (the first four letters of cardinality)
being a row. To remember the concept of degree, think of a wall
thermometer as a column (in other words, the temperature in

degrees as measured on a thermometer).

Relationships between the tables are defined to identify related
records. In this example, a relationship exists between the Customers
table and the Sales Reps table because each customer is assigned a
sales representative and each sales representative is assigned to one or
more customers. This relationship is reflected by the Sales Rep
field/column in the Customers table, shown in Figure 20.8. The values
in this column refer to a Sales Rep ID field contained in the Sales Rep

table (not shown). Additionally, a relationship would probably exist
between the Customers table and the Orders table because each order
must be associated with a customer and each customer is associated
with one or more product orders. The Orders table (not shown) would
likely contain a Customer field that contained one of the Customer ID
values shown in Figure 20.8.

Records are identified using a variety of keys. Quite simply, keys are a

subset of the fields of a table and are used to uniquely identify records.
They are also used to join tables when you wish to cross-reference
information. You should be familiar with three types of keys: