In SQL, an index is a data structure that improves the speed of data retrieval operations on a table. It allows the database engine to quickly locate the rows that match a specific query. In addition to indexing tables, SQL also supports indexing views, which are virtual tables derived from one or more underlying tables or views.

When it comes to views, SQL Server allows the creation of both clustered and non-clustered indexes. However, there is a limitation when it comes to non-clustered indexes on views.

Non-clustered indexes on views can only be created if the view satisfies certain conditions. These conditions are as follows:

1. The view must be an indexed view. An indexed view is a view that has been created with the WITH SCHEMABINDING option and has a unique clustered index defined on it. The WITH SCHEMABINDING option ensures that the underlying schema of the view remains unchanged, and any modification to the underlying tables requires updating the view as well.

2. The view must meet the requirements for indexed views. These requirements include not allowing the use of non-deterministic functions, no outer joins, and the use of the GROUP BY clause if aggregate functions are used.

Assuming the above conditions are met, you can create a non-clustered index on a view using the CREATE INDEX statement, similar to creating an index on a table. However, note that the index is created on the view and not on the underlying tables.

Here is an example of creating a non-clustered index on a view:

CREATE NONCLUSTERED INDEX IX_MyIndex ON dbo.MyView (Column1, Column2);

In the above example, IX_MyIndex is the name of the index, dbo.MyView is the name of the view, and (Column1, Column2) represents the columns on which the index is created.

So, while non-clustered indexes can be created on views in SQL Server, it is important to ensure that the view meets the necessary conditions for indexed views. By doing so, you can potentially improve the performance of queries that utilize the view.

#SQL #NonClusteredIndex