What Is a Primary Key in Oracle?

The values of a primary key must uniquely identify the row.

The values of a primary key in Oracle are always unique. Additionally, the values of a primary key cannot have null (missing or unknown) values, should not change over time and should be as short as possible.

Identification

A primary key is a constraint defined on a relational database table that prevents users from entering duplicate records into the table. A primary key constraint is defined using a set of columns in the database table that uniquely identifies the records stored in a table.

Features

In a table that stores employee information, a possible column that uniquely identifies the employee is Social Security number, because no two employees can have the same SSN. Employee name cannot be a primary key, because more than one person can have the same name.

Function

In an Oracle database, to define the column SSN as the primary key in the employee table, one would use the following syntax:

CREATE TABLE employee (

SSN NUMBER,

emp_name VARCHAR2(30),

salary number,

CONSTRAINT employee_pk PRIMARY KEY (SSN));

Effects

In the above example, when SSN is defined as the primary key on the table, Oracle will create a unique index for the SSN column in the employee table. This unique index will be used to enforce the rule that a duplicate value for SSN cannot be inserted into the table.

References

• Oracle FAQ

Resources

• TECH on the Net: Oracle/PLSQL: Primary Keys
• 1 Key Data: SQL Primary Key