What is an Alternate Key? Types, Differences, and Practical Examples

An alternate key in DBMS is a candidate key that uniquely identifies records but is not selected as the primary key. While a primary key is selected to uniquely identify each record in a table, an alternate key serves the same purpose but is not the one designated for primary use. 

Understanding the difference between candidate key and alternate key is crucial in designing efficient relational databases and ensuring data integrity.

What is an Alternate Key in DBMS? Characteristics and Importance

An alternate key in DBMS is a candidate key that uniquely identifies records but is not selected as the primary key. While a table can have multiple candidate keys, only one is designated as the primary key. 

The other candidate keys automatically become alternate keys, ensuring uniqueness but not serving as the primary identifier for the table.

Alternate keys provide flexibility. They offer alternative ways to reference data, especially when the primary key isn't practical. Understanding the difference between candidate and alternate keys helps in better structuring and querying relational databases.

Having a solid understanding of alternate keys is key to grasping their significance in database management. 

Now, let's explore the characteristics of alternate keys to better understand their role.

Characteristics of Alternate Key

An alternate key in DBMS serves as a unique identifier for records within a relational table, offering essential attributes that are crucial for data integrity and organization. These are the core characteristics of an alternate key:

• Uniqueness: An alternate key guarantees uniqueness within the table, ensuring that no two rows share the same value for this key. 

This is essential for maintaining data integrity, as it provides a means to differentiate every record distinctly, similar to a primary key.

• Non-Null Constraint: To maintain the accuracy and reliability of the data, an alternate key cannot contain null values. Every field under this key must have a value, which ensures the database enforces completeness and prevents incomplete records that could lead to inconsistencies.
• Candidate Key: The alternate key is essentially a candidate key that was not selected to be the primary key. While both candidate keys and primary keys are potential unique identifiers, the alternate key is one of the remaining keys, still holding the capability to uniquely identify records but not chosen as the default key.

Example:

Consider the following Employees table:

In this example:

• EmployeeID is the Primary Key (PK) as it uniquely identifies each record.
• Email is an Alternate Key (AK) because it also contains unique values but was not chosen as the primary key.

These characteristics highlight the structural integrity of an alternate key in ensuring that relational databases maintain consistency, completeness, and proper data identification without redundancy.

Before diving deeper into the practical implications, it's essential to examine the unique characteristics that set alternate keys apart in a relational database.

Importance of Alternate Key

The alternate key plays a pivotal role in DBMS, contributing to the overall design, flexibility, and functionality of the database. Here’s a deeper look at its importance:

• Data Retrieval Flexibility: An alternate key allows for flexible data retrieval by enabling searches or joins using alternative identifiers, like email or phone number, making queries more efficient when the primary key isn’t easily accessible.
• Contribution to Normalization: Alternate keys support database normalization by reducing redundancy and ensuring a logically structured schema, helping maintain data integrity and avoid anomalies such as update, insertion, or deletion issues.
• Adapting to Business Logic: Alternate keys often align with real-world business logic, such as email or product serial numbers, making the database more practical and user-friendly compared to system-generated primary keys like auto-incrementing IDs.

The alternate key in DBMS provides significant advantages in terms of flexibility, efficiency, and aligning the database with business requirements. It offers an alternative method of accessing and managing data while preserving the database’s structure and functionality. 

Also Read: What Are The Types of Keys in DBMS? Examples, Usage, and Benefits

Now that the characteristics are clear, let's move on to discuss why alternate keys are so crucial in ensuring data integrity and flexibility in database operations.

How to Create an Alternate Key in DBMS??

In a DBMS, an alternate key is a candidate key not selected as the primary key but that could uniquely identify records. A candidate key is any field or combination of fields that can serve as a unique identifier. All alternate keys are candidate keys, but only those not chosen as the primary key are considered alternate keys.

To create an alternate key in a DBMS, the UNIQUE constraint is used to ensure that values in a column or a set of columns are unique across all rows. This constraint can be applied to any column (or set of columns) that is not the primary key but is still a candidate key.

Let us now have a look at the necessary steps to create an Alternate Key: 

Step 1: Define the Primary Key

Start by defining the primary key for your table, which is the column or combination of columns that will uniquely identify each row.

Step 2: Define Alternate Keys

To create an alternate key, apply the UNIQUE constraint to the other candidate keys. This ensures that the values in these columns are unique, but they are not selected as the primary key.

Example

Consider the following tables: Product and Customer. In this example, ProductID is the primary key in the Product table, and SKU is defined as an alternate key. Similarly, CustomerID is the primary key in the Customer table, and Email is defined as an alternate key.

Step 1: Define the Primary Key

CREATE TABLE Product (
    ProductID INT PRIMARY KEY,        -- Primary Key
    ProductName VARCHAR(255),
    SKU VARCHAR(100),                 -- Alternate Key
    Price DECIMAL(10, 2)
);

CREATE TABLE Customer (
    CustomerID INT PRIMARY KEY,       -- Primary Key
    Email VARCHAR(255),                -- Alternate Key
    Name VARCHAR(100),
    Address VARCHAR(255)
);

Step 2: Apply the UNIQUE Constraint to Define Alternate Keys

ALTER TABLE Product
ADD CONSTRAINT unique_sku UNIQUE (SKU);      -- Define SKU as an Alternate Key

ALTER TABLE Customer
ADD CONSTRAINT unique_email UNIQUE (Email);  -- Define Email as an Alternate Key

In this example:

• ProductID is the Primary Key in the Product table.
• SKU is an alternate key.
• CustomerID is the Primary Key in the Customer table.
• Email is an alternate key.

Step 3: Insert Data and Test Constraints

-- Inserting data into the Product table
INSERT INTO Product (ProductID, ProductName, SKU, Price) 
VALUES (1, 'Saree', 'SKU98765', 1200.50);

-- Attempt to insert a duplicate SKU (should fail)
INSERT INTO Product (ProductID, ProductName, SKU, Price) 
VALUES (2, 'Kurti', 'SKU98765', 800.00);  -- This will fail due to the UNIQUE constraint on SKU

-- Inserting data into the Customer table
INSERT INTO Customer (CustomerID, Email, Name, Address) 
VALUES (1, 'rajesh@tata.com', 'Rajesh Kumar', 'Bangalore, Karnataka');

-- Attempt to insert a duplicate Email (should fail)
INSERT INTO Customer (CustomerID, Email, Name, Address) 
VALUES (2, 'rajesh@tata.com', 'Suman Rani', 'Hyderabad, Telangana');  -- This will fail due to the UNIQUE constraint on Email

Outcome:

1. The first insert into the Product table will succeed because SKU is unique.
2. The second insert into the Product table will fail because SKU 'SKU98765' is already used, thanks to the UNIQUE constraint on SKU.
3. The first insert into the Customer table will succeed, but the second insert will fail because Email 'rajesh@tata.com' already exists, ensuring uniqueness through the alternate key.

This ensures data integrity and consistency, preventing duplicate entries for crucial attributes like SKU or Email in the respective tables.

Also Read: Top 27 SQL Projects in 2025 With Source Code: For All Levels

Once you know how to create an alternate key, it's important to grasp the differences between various types of database keys. Let’s explore the distinctions between primary, candidate, and alternate keys in SQL.

Database Keys in SQL: Difference and Examples

In SQL databases, keys are crucial in ensuring data integrity and optimizing database operations. These keys are used to uniquely identify records in a table, establish relationships between tables, and enforce the accuracy of data. 

The main types of keys in SQL are candidate keys, primary keys, alternate keys, and foreign keys. Each type of key serves a different role in maintaining the structure of the database.

The types of keys in SQL are as follows:

• Candidate Key: A set of columns in a table that can uniquely identify any record in that table. A table can have one or more candidate keys.
• Primary Key: A unique key selected from the candidate keys to uniquely identify records in a table. Each table can have only one primary key.
• Alternate Key: A candidate key that was not selected as the primary key. Alternate keys are still unique identifiers, but they serve as backups to the primary key.
• Foreign Key: A column or set of columns that links one table to another. A foreign key is used to maintain referential integrity by ensuring that the value in the foreign key column matches a primary or unique key in another table.

Also Read: Primary Key in SQL Database: What is, Advantages & How to Choose

Next, let’s break down the difference between candidate and alternate keys to gain a clearer perspective on their respective roles in database design.

What is the Difference Between Candidate Key and Alternate Key?

Candidate Key and Alternate Key are both used to uniquely identify records in a table. The primary distinction between the two is their selection for the primary key.

• Candidate Key: Candidate keys can be a single column or a combination of columns that uniquely identify a row in a table. A table can have multiple candidate keys.
• Alternate Key: A candidate key that was not chosen as the primary key. All alternate keys are candidate keys, but not all candidate keys are alternate keys.

Example to Illustrate the Difference

Consider a Student table:

Here:

• StudentID, RollNumber, and Email can all uniquely identify a student.
• If StudentID is selected as the primary key, then RollNumber and Email become alternate keys, as they are still unique identifiers but were not chosen as the primary key.

Here is a quick tabular recap of the differences between candidate and alternate keys: 

Now, let’s dive into the relationship between primary and alternate keys and understand how the primary key serves as the main identifier while the alternate key provides a backup identifier.

What is the Difference Between Primary Key and Alternate Key?

The primary key is the most critical unique identifier in a table, chosen from the set of candidate keys. The alternate key, on the other hand, is a backup unique identifier that was not selected as the primary key but still ensures uniqueness.

• Primary Key: The main key selected from the candidate keys to uniquely identify records in a table. It cannot have NULL values and must be unique.
• Alternate Key: A candidate key that is not selected as the primary key. It also ensures uniqueness but is used as a backup.

Here is a quick table that captures the differences between primary and alternate keys: 

Understanding the differences between these keys helps in designing a robust database structure that maintains data integrity and ensures efficient querying.

Now that you've mastered key concepts in DBMS, it’s time to take the next step in your career journey. Let’s delve into how upGrad's offerings can further enhance your database management expertise and propel your career forward.

How Can upGrad Help You Advance Your Career?

upGrad’s specialized 200+ programs are designed to enhance various skills, including DBMS expertise, a key skill for success in today’s tech-driven job market. Whether you're starting out or refining your knowledge, these programs offer targeted training in database design, SQL, optimization, and performance tuning.

Here are some top courses to help you excel in your DBMS career:

• Advanced SQL: Functions and Formulas
• Introduction to Database Design with MySQL
• Executive Diploma in Data Science & AI with IIIT-B
• Post Graduate Certificate in Data Science & AI (Executive)

Struggling to find the right DBMS course or career path? Connect with upGrad’s counselors or visit your nearest upGrad career center for personalized guidance to enhance your skills and reach your career goals!

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