Alternate Key in DBMS (original) (raw)

Last Updated : 23 Jul, 2025

Keys play an important role in organizing and accessing data in the database management system. There are many key types of keys but primary keys are mostly discussed because of their unique identification properties, we can identify the attributes of an element with the primary key, but there are also alternative keys. Which are not that extensively discussed. But are unique and present in the database table. They present as similar and alternative to the primary keys. In this article, we will understand what is primary key. Its characteristics, advantages, and examples.

What is the Alternative Key?

As the name suggests, the alternative key is the key in the database table which is not selected as a primary key. So it's an additional key similar to the primary key with which we can access data in the table. It can uniquely identify attributes of each element in the row Of the table. But, because there can be multiple candidate keys all cannot be selected. So, the remaining keys which are not selected as primary keys are called alternative keys. The main reason for having an alternate key is to provide more additional means with the help of. Then we can access data in the table in case the primary key is not applicable. It makes sure that there are more ways than the primary key.

Characteristics of Alternative Key

• **Uniqueness: An alternate key must be unique, which means that no two elements can have the same values. For an alternate key this is important because it ensures that each record of the table can be uniquely identified by the Alternately key Which makes the alternate key ideal as a candidate key and the primary key.
• **Not-Null constraint: Because an alternate key has the potential to be a primary key. So alternate key cannot have null values. This means that each record in the table must have a value for the alternate key columns. This is because null records cannot be identified and may cause problems with the Integrity of the database.
• **Candidate key: Candidate key means that the alternate key is a possible candidate for becoming our primary key. If the chosen primary key has some problems and or does not fulfill requirements. An alternate key has all the properties that are necessary to serve as the primary key. However, due to design constraints Or decisions, it serves as an alternative.

Examples of Alternate Key

Example 1: Student Database

Imagine a database table named STUDENTS that contains the following columns:

• Student_ID: A unique identifier for each student.
• Email: The student’s email address.
• Phone_Number: The student’s phone number.

In this table, both Email and Phone_Number could serve as candidate keys because they are unique to each student. If Student_ID is chosen as the Primary Key, then Email and Phone_Number become Alternate Keys. They can be used to uniquely identify a student if the Student_ID is not known.

Example 2: Employee Database

Consider an EMPLOYEES table with the following attributes:

• Employee_Number: A unique number assigned to each employee.
• SSN: Social Security Number, unique to each individual.
• Email: The employee’s work email address.

If Employee_Number is the Primary Key, SSN and Email would be Alternate Keys because they can also uniquely identify an employee in the database.

The EMPLOYEES table could look like this: