Difference Between Clustered and Non-Clustered Indexes in SQL Server

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Difference Between Clustered and Non-Clustered Indexes in SQL Server

Indexes in SQL Server are used to improve the speed of data retrieval. There are two main types:

  1. Clustered Index
  2. Non-Clustered Index

Understanding the differences between them helps in designing efficient database schemas and optimizing query performance.

 


Clustered Index

  • A clustered index determines the physical order of data in the table.
  • A table can have only one clustered index.
  • The table data is sorted and stored in the order of the clustered index key.
  • Often created on the primary key column by default.

Example:

CREATE CLUSTERED INDEX IX_Employees_Id

ON Employees(Id)

Analogy: Think of a phone book arranged alphabetically by last name — the data itself is in sorted order.

 

Non-Clustered Index

  • A non-clustered index creates a separate structure from the table data.
  • Points to the location of the data using a reference to the clustered index key or row identifier.
  • A table can have multiple non-clustered indexes.
  • Useful for searching or filtering on columns other than the primary key.

Example:

CREATE NONCLUSTERED INDEX IX_Employees_Department

ON Employees(Department)

Analogy: Like an index in a book — it points you to the actual page where the topic exists.

 

Key Differences

Feature

Clustered Index

Non-Clustered Index

Storage

Sorts and stores actual table rows

Stores a separate copy with pointers

Number per table

Only one

Multiple (up to 999 in SQL Server)

Speed

Faster for range-based queries

Faster for lookup/search on non-key columns

Default on

Primary key

Foreign key, frequently queried fields

Data Access

Direct (data is part of the index)

Indirect (requires lookup to get full row)

 

When to Use

  • Clustered Index: Use when querying ranges or sorting on the column (e.g., dates, IDs).
  • Non-Clustered Index: Use for searching/filtering on non-primary columns (e.g., name, status, category).

 

Summary

  • A clustered index sorts and stores the actual table data in order — one per table.
  • A non-clustered index is a separate structure that stores the indexed column and a pointer to the data — many per table.

Efficient indexing is key to boosting query performance and minimizing I/O operations in large databases.

 

 

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