Can you explain the differences between clustered and non-clustered indexes in SQL Server, and when would you use each type?

Clustered vs. Non-Clustered Indexes in SQL Server: Explained

In SQL Server, clustered and non-clustered indexes play crucial roles in optimizing database performance. The main difference between the two lies in how the data is physically stored on disk.

Clustered Index: A clustered index determines the physical order of the rows in a table. In other words, the actual data rows are stored in the order of the clustered index key. Each table can have only one clustered index, and it directly affects the way data is stored on disk.

Non-Clustered Index: In contrast, a non-clustered index does not change the physical order of the table rows. Instead, it creates a separate structure to store the index key values along with pointers to the actual data rows. A table can have multiple non-clustered indexes, which can provide flexibility in optimizing query performance.

When to Use Each Type: The choice between clustered and non-clustered indexes depends on the specific use case.
- Use a clustered index when the table is frequently queried using range searches or when you need to retrieve a large number of rows in a sorted order.
- Non-clustered indexes are ideal for columns frequently used in queries to locate specific rows quickly, or to cover queries containing columns not included in the clustered index.

In conclusion, understanding the differences between clustered and non-clustered indexes in SQL Server, and knowing when to use each type, is essential for optimizing database performance and query processing efficiency for your applications.

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