What are the SQL Alternate Keys?

Badrinarayan M 03 Jul, 2024
4 min read


Keys are an important part of SQL, and the primary, foreign, and candidate keys hold a significant value. The alternate key holds are crucial, yet we often overlook their importance. They are used to design our database, Ensure the integrity of our data, and create an efficient way to retrieve records. Intriguing right? Further in this article, we will discuss SQL Alternate Key in detail.

SQL Alternate Keys


  • Understand the purpose and basic syntax of the alternate key.
  • Identify scenarios where the Alternate key can be applied effectively.
  • Implement the alternate key to avoid inconsistency and depletion of integrity.
  • Use cases of alternate keys and the advantages of having an alternate key.

What is an alternate key?

In simple terms, an Alternate key is a candidate key that is not a primary key. Everyone knows that the primary key uniquely identifies each record in the table. The alternate key also uniquely identifies the records in the table and holds the same purpose. 

Important aspects of alternate key

  • Data Integrity – This ensures that each row in our table is unique
  • Data retrieval –  Provides additional unique columns that can be used for retrieving data efficiently
  • Redundancy Elimination – By ensuring that columns are unique, we reduce redundancy. 

Syntax for creating alternate keys

Creating an alternate key is making columns with unique constraints. 

ALTER TABLE table_name
ADD CONSTRAINT constraint_name UNIQUE (column1, column2, ...);

Creating alternate keys

Let’s first create a table to learn about alternate keys. 

CREATE TABLE Employees (
    Email VARCHAR(255) NOT NULL,
    FirstName VARCHAR(50),
    LastName VARCHAR(50)
SQL Alternate Keys

Here, we can see that EmployeeID is our primary key. We also know two employees can’t have the same email address and SSN. Hence, we have to implement unique constraints for them. 


ADD CONSTRAINT unique_email UNIQUE (Email);
SQL Alternate Keys

The above code will make Email and SSN unique. This makes them an alternate key to the table Students.

Use cases of Alternate key

  • Unique user identifiers – From the above example, Email and EmployeeID can be used to identify unique employees in user management systems. This enhances the retrieval when EmployeeID is not known,
  • Inventory System – Product codes and serial numbers can both uniquely identify items.
  • Student Records – StudentID and PhoneNumber can both act as unique identifiers.

Also read: SQL: A Full Fledged Guide from Basics to Advanced Level

Some samples with Employee Table

Let’s try inserting some sample data into our Employee table. 

INSERT INTO Employees (EmployeeID, SSN, Email, FirstName, LastName) 
VALUES (1, '123-45-6789', '[email protected]', 'John', 'Doe');

INSERT INTO Employees (EmployeeID, SSN, Email, FirstName, LastName) 
VALUES (2, '987-65-4321', '[email protected]', 'Jane', 'Doe');
SQL Alternate Keys

From the above two codes, we can see there is no problem, and the code is getting executed. 

INSERT INTO Employees (EmployeeID, SSN, Email, FirstName, LastName) 
VALUES (3, '123-45-6789', '[email protected]', 'John', 'Smith');
SQL Alternate Keys

We can see that this fails as the SSN must be unique; let’s try a similar thing with email

INSERT INTO Employees (EmployeeID, SSN, Email, FirstName, LastName) 
VALUES (4, '555-55-5555', '[email protected]', 'Johnny', 'Doe');
SQL Alternate Keys

We can see that this fails as the email should be unique, [email protected] is already present in our database. 

Also read: SQL For Data Science: A Beginner Guide!

Advantages of Using Alternate Keys

  • Enhanced Data Integrity: Ensures that critical columns remain unique.
  • Flexibility in Data Retrieval: Allows for multiple unique columns for querying.
  • Better Indexing: Improves search efficiency with multiple unique indexes.


Alternate key plays an important role in the SQL and database management systems. We should try implementing and using them to enhance integrity and flexibility. We can ensure a robust and flexible database with an understanding of alternate keys. 

Frequently Asked Questions

Q1. What is an alternate key in SQL?

Ans. Any candidate key which is not a primary key is an alternate key.  

Q2. How is an alternate key different from a primary key?

Ans. There is less difference in the functionality of alternate and primary keys. A candidate key, which is not a primary key, is an alternate key. Both identify records uniquely.

Q3. What is a candidate key?

Ans. A candidate key is a minimal attribute that uniquely identifies a record.

Q4. What is a super key?

Ans. A super key is any combination of attributes uniquely identifying a record, including candidate keys.

Badrinarayan M 03 Jul, 2024

Frequently Asked Questions

Lorem ipsum dolor sit amet, consectetur adipiscing elit,

Responses From Readers