SQL Unique index

A unique index in SQL is a database structure that enforces uniqueness on one or more columns in a table. It ensures that no duplicate values can be inserted into the specified column(s) of a table. A unique index can be created on a single column or on a combination of multiple columns, depending on the requirements of the database schema.

The purpose of a unique index is to provide a fast and efficient way to enforce data integrity by preventing duplicate entries in a table. It guarantees that each value in the indexed column(s) is unique across all rows in the table. This can be particularly useful when dealing with primary keys, where a unique identifier is required for each record.

When a unique index is created on a column or a set of columns, the database system automatically checks for uniqueness whenever a new row is inserted or an existing row is updated. If a duplicate value is detected, the database will raise an error and reject the operation, ensuring the integrity of the data.


The syntax for creating a unique index varies slightly depending on the database management system (DBMS) you are using. However, the basic structure remains the same. Here is a general syntax:

ON table_name (column1, column2, ...);

Let’s break down the syntax components:

CREATE UNIQUE INDEX is the statement used to create a unique index.
index_name is a name you choose to identify the index. It should be unique within the database.
table_name refers to the name of the table on which the index is being created.
(column1, column2, …) represents the column or columns on which the uniqueness constraint is applied.


Here’s an example of creating a unique index on a single column in SQL using the CREATE INDEX statement:

CREATE UNIQUE INDEX idx_employee_email 
ON employees (email);

In the above example, a unique index named “idx_employee_email” is created on the “email” column of the “employees” table. This index ensures that each email address stored in the “email” column is unique, preventing duplicate email entries in the table.

Unique indexes can also be created on multiple columns, providing a combined uniqueness constraint. Here’s an example:

CREATE UNIQUE INDEX idx_employee_name_department 
ON employees (first_name, last_name, department);

In this case, the unique index named “idx_employee_name_department” is created on the combination of “first_name,” “last_name,” and “department” columns of the “employees” table. It ensures that each combination of these columns is unique, preventing duplicate entries based on these criteria.

It’s important to note that unique indexes impose a constraint on the data, which means they come with some trade-offs. Inserting or updating rows that violate the unique constraint will fail, requiring proper handling of errors in your application logic. Additionally, unique indexes can impact the performance of insert and update operations, as the database needs to perform additional checks to ensure uniqueness.

However, despite these considerations, unique indexes are valuable tools for maintaining data integrity in SQL databases. They provide a reliable means of enforcing uniqueness on columns or combinations of columns, ensuring that data remains consistent and accurate throughout the database.