SQL Server Concepts - Indexes

Overview

Indexes are used in SQL Server to improve the performance of data retrieval operations on tables. They are structures that consist of one or more columns of a table and provide a quick access path to the table's data. An index can be created on one or more columns of a table.

Syntax

The syntax for creating an index is as follows:

CREATE [UNIQUE] [CLUSTERED | NONCLUSTERED] INDEX index_name
ON table_name (column_name [ASC | DESC], ...)

Example

CREATE INDEX idx_name
ON employees (last_name, first_name)

Output

The output of the above example is an index named "idx_name" created on the "employees" table with columns "last_name" and "first_name".

Explanation

An index is created on a table to speed up data retrieval operations. SQL Server uses a B-tree structure to store the index data, which allows for fast access to the data. The type of index (clustered or nonclustered) determines how the data is stored and how it is accessed.

Clustered indexes are created on the primary key column(s) of a table and determine the physical order in which the data is stored. Nonclustered indexes are created on columns other than the primary key and provide a separate access path to the data.

Use

Indexes are used to speed up data retrieval operations on tables. They are especially useful for tables with large amounts of data or tables that are frequently queried. Indexes can also be used to enforce unique constraints on a table.

Important Points

• Indexes should only be created on columns that are frequently used in queries.
• Too many indexes on a table can actually slow down query performance.
• Clustered indexes are created on the primary key column(s) of a table.
• Nonclustered indexes are created on columns other than the primary key.

Summary

Indexes are used in SQL Server to improve the performance of data retrieval operations on tables. They are structures that consist of one or more columns of a table and provide a quick access path to the table's data. Indexes can be created on one or more columns of a table, and they can be classified as clustered or nonclustered. Clustered indexes are created on the primary key column(s) of a table and determine the physical order in which the data is stored. Nonclustered indexes are created on columns other than the primary key and provide a separate access path to the data.