What is the difference between a clustered and non-clustered index in SQL Server?

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Answered by suresh

Clustered vs Non-Clustered Index in SQL Server

When it comes to indexing in SQL Server, there are two main types of indexes: clustered and non-clustered indexes. Understanding the difference between these two types of indexes is crucial for optimizing database performance.

Clustered Index:

A clustered index is a type of index in SQL Server that physically organizes the data in the table based on the index key. This means that the data rows in the table are stored in the order of the clustered index key. Each table can have only one clustered index, as it determines the physical order of the data rows.

When a table has a clustered index, the data rows are stored in the order specified by the index key, which can improve read performance for range queries and data retrieval operations. However, inserting and updating data in a table with a clustered index can be slower compared to a table without a clustered index.

Non-Clustered Index:

A non-clustered index is a type of index in SQL Server that does not affect the physical order of the data in the table. Instead, it creates a separate data structure that points to the actual data rows based on the index key. Unlike clustered indexes, a table can have multiple non-clustered indexes, allowing for efficient querying on different columns.

Non-clustered indexes are useful for speeding up query performance for SELECT, JOIN, and WHERE clauses that do not match the clustered index key. However, they may result in slower data retrieval for range queries or data modification operations compared to clustered indexes.

In summary, the main difference between a clustered and non-clustered index in SQL Server is in how the data is physically organized. Clustered indexes store data rows in the table order specified by the index key, while non-clustered indexes create a separate data structure to point to the actual data rows based on the index key. Each type of index has its own advantages and trade-offs, and the choice between clustered and non-clustered indexes depends on the specific requirements of the database and query workload.

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