Programming Oracle® Triggers and Stored Procedures, Third Edition

3.2. UNIQUE

In this section we use a table that stores student information. The table description is:

Name                           Null?    Type
------------------------------ -------- ------------
STUDENT_ID                              VARCHAR2(10)
STUDENT_NAME                            VARCHAR2(30)
COLLEGE_MAJOR                           VARCHAR2(15)
STATUS                                  VARCHAR2(15)
STATE                                   VARCHAR2(2)
LICENSE_NO                              VARCHAR2(30)

The UNIQUE constraint is applied to a column, or set of columns, to enforce the following rule: If a value exists, than that value must be unique.

This definition sounds similar to a PRIMARY KEY constraint. The following is a comparison between the primary key and unique constraints.

• A table can have just one primary key constraint but a table can have more than one unique constraint.

• A column that is part of a primary key can never have ...