Adventures in Machine Learning

Mastering SQL Server Unique Indexes: Preventing Duplicates with Precision

SQL Server Unique Index Overview

If youre working with SQL servers, then chances are you’ve come across unique indexes before. Despite its importance, not many database administrators completely grasp the full concept behind unique indexes.

What is a unique index? A unique index is a database constraint that forbids duplicate values from being entered into a database.

This means that within a column or a set of columns, each value must be unique. This is enforced by the server and needs to be adhered to, otherwise you will receive an error if you try to insert duplicate data.

Single Column Unique Index

A single column unique index is a database constraint that prohibits duplicate values within a single column. This means that each record in the column will have a unique value.

In the case of a database consisting of employees, the ‘EmployeeID’ column would likely be an example of a single column unique index. This is because each employee has their own unique identifier.

Multiple Column Unique Index

A multiple column unique index is a database constraint that prohibits duplicate values within multiple columns. This means that the combination of values across these columns must be unique.

For example, if we create an orders table for a company, the combination of ‘OrderNumber’ and ‘CustomerID’ columns must be unique to prevent any duplicate orders from being entered.

Error Prevention for Duplicate Values

Unique indexes exist to prevent duplicate values from being entered into a database. However, if you try to insert, or update, duplicate data, the server will provide an error.

This can be an issue if you don’t manage your data or if something goes wrong which could lead to duplicate data being entered.

Clustered and Non-Clustered Unique Index

Unique indexes can also be defined as either clustered or non-clustered. Clustered unique indexes organize data in the table using the structure of the index itself.

Meaning that the index is physically based on the data. While non-clustered unique indexes separate the index and the data, storing the index separately.

Creating a SQL Server Unique Index

Creating a unique index on your database can help prevent duplicates which can wreak havoc on processes relying on correct data to function. Here’s how to create one:

Syntax for Creating Unique Index

The syntax for creating a unique index is as follows:


CREATE UNIQUE INDEX [IndexName] ON [Table] ([ColumnName]);

— Example

CREATE UNIQUE INDEX ui_EmployeeID ON dbo.Employees (EmployeeID)


Unique Index for One Column Example

In the case of a single column unique index, the easiest way to create it is within a table creation script. This example outlines how to create a table Employees with it’s unique index EmployeeID.









CREATE UNIQUE INDEX ui_EmployeeID ON Employees (EmployeeID);


Checking Duplicate Values Before Creating Unique Index

Prior to creating your unique index, run the following exercise to verify there are no duplicates in the column you’re targeting for indexing. “`

SELECT [ColumnName], COUNT(*) AS [Count]

FROM [Table]

GROUP BY [ColumnName]



If the query returns any rows of data, that means there are duplicates that need to be fixed before a unique index can be created.

Unique Index for Multiple Columns Example

Creating a unique index for multiple columns follows a similar process as single column indexes. Here’s an example:








CREATE UNIQUE INDEX ui_Orders ON Orders (OrderNumber, CustomerID);


Error Prevention for Duplicate Values with Unique Index

Once a unique index is added to a table, any duplicates entered into that column will be rejected by the server. This serves as an alert to the user that the data being entered may be incorrect.


In conclusion, unique indexes are an essential aspect of working with SQL servers. They help ensure the database’s integrity, prevent errors, and speed up data queries.

The creation of a unique index requires proper planning and checking of the data to ensure no duplicates exist before the index is made.

SQL Server Unique Index and Null

When working with SQL servers, NULL values and their interaction with unique indexes can create confusion. In this article, we’ll explore the relationship between nullable columns, NULL values and unique indexes.

Additionally, we’ll discuss how unique constraints relate to unique indexes and what their objective is.

Definition of NULL

NULL is a special value in SQL that represents missing or unknown data. When a column is nullable, it can have NULL values, meaning no data exists in that column for some records.

Unique Index on Nullable Column and NULL Values

A nullable column that is part of a unique index can have NULL values in it. However, duplicates of NULL values are allowed in a unique index.

This means that if a column is nullable and contains NULL, it isn’t considered a duplicate value under the unique index constraint. This behavior is different from that of non-nullable columns.

For example, assuming we have a table called ‘Person’ with the following schema:








If we set up a unique index on the ‘LastName’ column, the index will not prevent a record with a NULL value in ‘LastName’ column to be created. This is because according to SQL standards, NULL is not equal to any other value, including NULL values.

Unique Constraint and Unique Index

A unique constraint is a similar concept to a unique index in SQL Server. It is a database constraint that requires all values in a column or set of columns to be unique.

It is supported by a unique index, but it is a database object independent of the index. A unique constraint is also a data integrity tool that enforces uniqueness in a table.

One advantage of using a unique constraint over a unique index is that a unique constraint can be named. Also, if you want to enforce uniqueness across multiple columns, you only need to create a unique constraint, not a unique index for each column.

Objective of Unique Constraint

The objective of a unique constraint in SQL is the same as that of a unique index, which is to ensure the integrity of data in a table by preventing duplicate values in a column or set of columns. Unique constraints are especially useful if you want to maintain the uniqueness of data even when the database schema changes.

For example, if you want to enforce uniqueness for a combination of columns, you only need to create one unique constraint. In contrast, if you were to use a unique index, you would need to create a unique index for each column.

In addition, a unique constraint also makes it easier to read the purpose of the constraint from the schema definition. Naming the constraint can also make it easier to refer to in error messages or code.

Summary of SQL Server Unique Index

A unique index in SQL Server is a database constraint that forbids duplicate values from being entered into a database. It can be created for a single column or a set of columns that are declared to be unique across different rows in a table.

If the index is created on a non-nullable column, then any attempt to insert duplicate values will result in an error.

When it comes to nullable columns, the unique index will accept any values including NULL, since NULL is not considered a regular value.

Unique indexes are useful for maintaining the integrity of data, especially when it comes to primary keys. They can help speed up data queries and avoid problems related to having duplicate data.

In addition to unique indexes, unique constraints are another database tool for ensuring uniqueness. Unique constraints and unique indexes share the objective of ensuring uniqueness of data in a table.

However, unique constraints are also more flexible in that they can be named and enforce uniqueness across multiple columns without needing multiple unique indices.

Understanding the differences between unique indexes, unique constraints, and NULL values in nullable columns can help you design effective database schemas and maintain data integrity in your SQL server.

In conclusion, SQL Server unique indexes ensure that no duplicates are entered into a database, and they can be created for both single and multiple columns, with duplicates of NULL values for nullable columns being allowed. Unique constraints serve the same purpose of ensuring data integrity, but they are flexible in terms of naming and enforcing uniqueness across multiple columns.

Understanding the differences between unique indexes and constraints can help with designing effective database schemas. Takeaways include planning and checking data before creating an index, and the use of unique constraints to maintain uniqueness even with changing schema.

Ultimately, data integrity is crucial, and unique indexes and constraints can help maintain the quality of data in SQL servers.

Popular Posts