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 ...
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