Basic Oracle Concepts and Programming Question:

An index associated with a constraint because this constraint is required to have an index. There are two types of constraints are required to have indexes: UNIQUE and PRIMARY KEY. When you defines a UNIQUE or PRIMARY KEY constraint in a table, Oracle will automatically create an index for that constraint. The following script shows you an example:

CREATE TABLE student (id NUMBER(5) PRIMARY KEY,
first_name VARCHAR(80) NOT NULL,
last_name VARCHAR(80) NOT NULL,
birth_date DATE NOT NULL,
social_number VARCHAR(80) UNIQUE NOT NULL);
Table created.

SELECT index_name, table_name, uniqueness
FROM USER_INDEXES WHERE table_name = 'STUDENT';
<pre>INDEX_NAME TABLE_NAME UNIQUENES
----------------------- --------------------- ---------
SYS_C004123 STUDENT UNIQUE
SYS_C004124 STUDENT UNIQUE</pre>
The result confirms that Oracle automatically created two indexes for you.