SQL Server Clustered and Non-Clustered Indexes - An Overview

Indexes are essential for efficient data retrieval in SQL Server. In this overview, we'll explore the concepts of clustered and non-clustered indexes, their differences, and when to use each type of index for optimizing query performance. We'll also provide sample code snippets to help you understand the implementation of these indexes.

What Are Indexes?

Indexes in SQL Server are database objects that enhance the speed of data retrieval operations on database tables. They work similarly to the index in a book, allowing SQL Server to quickly locate the rows that satisfy a query.

Clustered Index

A clustered index determines the physical order of data rows in a table. Each table can have only one clustered index, and it impacts the organization of the entire table. Typically, it is created on the primary key column, but it can be on any column with a unique constraint. Here's how to create a clustered index:

-- Create a clustered index
CREATE CLUSTERED INDEX IX_ClusteredIndex
ON TableName (PrimaryKeyColumn);
    

Non-Clustered Index

A non-clustered index creates a separate structure to optimize query performance, but it does not affect the physical order of data rows. Multiple non-clustered indexes can exist on a single table. To create a non-clustered index:

-- Create a non-clustered index
CREATE NONCLUSTERED INDEX IX_NonClusteredIndex
ON TableName (IndexedColumn);
    

When to Use Clustered vs. Non-Clustered Indexes

Clustered indexes are best suited for columns that are frequently used in JOIN operations and for ensuring data integrity. Non-clustered indexes should be used for columns commonly used in WHERE clauses to improve query performance. Choose the right index type based on your specific use case.

What's Next?

As you become more familiar with SQL Server clustered and non-clustered indexes, explore advanced topics such as covering indexes, included columns, and index maintenance strategies to optimize your database's performance.