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RELATIONAL DATABASES 23

Customer Address, Customer City, Customer State, Customer ZIP Code, and Customer
Phone Number and Customer Email. Each row of a table represents an instance (or a record)
of the entity. That is, one specific row in the Customer Table provides all the data values for
one specific customer. A primary key attribute is a unique identifier of each instance of an
entity. For example, the primary key of the Customer Table is Customer ID. Customer Name
would not be an appropriate primary key unless Jesse is absolutely sure that he does not and
will not ever have customers with the same name, which is a practical impossibility.
An important property of the relational database is that it represents relationships
between entities by including values stored in the columns of the corresponding tables. For
example, the Cash Receipt Table may include attributes such as Cash Receipt #, Date,
Amount, and Customer ID. By including the Customer ID in the Cash Receipt Table we
can link the Cash Receipt Table and the Customer Table to obtain data for decision-mak-
ing purposes. For example, we can easily link a cash receipt to its associated customer.
The Customer ID is called a foreign key in the Cash Receipt Table. A foreign key is
an attribute appearing in one table that is a primary key in another table. Foreign keys are
used to link tables. Another example of using a foreign key would be including "Item #"
as an attribute in the Sale Table. "Item #" in the Sale Table then would be a foreign key
because (assuming continuity of table design) it is the primary key of the Inventory Table.
We can use the cardinalities in the REA diagram from Chapter I to determine these
relationships between the entities in terms of a relational database. We do this by referring
to the maximum cardinality on each side of the relation in the REA diagram. Since we
know that a maximum cardinality can be a 1 or an N, we have three possible relationships.
A one-to-one (1:1) relationship occurs when a record (row) in a table relates to a record in
another table once and only once. A one-to-many (1 :N) relationship occurs when a record
in a table can relate to several records in another table. A many-to-many (N:N) relation-
ship occurs when several records in a table relate to several records in another table.

Basic Requirements of Tables

The relational database approach imposes some requirements on the structure of tables. If
these basic requirements are not fulfilled or if data redundancy exists in a database, prob-
lems (called anomalies) may occur. The requirements include the following:

• The Entity Integrity Rule: Each table in a relational database must have a primary
key attribute, and it must have a value for each record (i.e., it cannot be null).
• The Referential Integrity Rule: We use foreign keys to link tables so the corre-
sponding values must match. A value for a foreign key attribute must either be null
or match one of the data values that correspond to the value of a primary key attribute
~ l
in another table. Y
• Each attribute must be uniquely named
• Values of a specific attribute must be of the same type. In other words, a column
must be designated as text, numeric, currency, etc., and the data must be of only that
type. For example, data in a column designated as currency must all be in dollars and
cents format.
• Each attribute (column) of a record (row) must be single-valued. This requirement
forces us to create a relationship table for each many-to-many relationship.