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Non-clustered index in SQL Server

Non-clustered index
A non-clustered index has a structure separate from data row, it means non-clustered order of the row does not match the physical order of the actual data. If we talk about the B-TREE structure, in the non-clustered index, the leaf pages of the index do not contain any actual data, but instead, contain pointers to the actual data. These pointers would point to the clustered index data page where the actual data exists (or the heap page if no clustered index exists on the table).

Non-clustered indexes (also called “indexes”) are the normal indexes that are created on your tables. SQL Server supports 999 non-clustered indexes per table.

Each non-clustered index can contain up to 1023 included columns. Columns of the TEXT, NTEXT and IMAGE data types cannot be included either.

There are more things that could be put in here, but this will get you a good start. If you think of anything that I’ve missed please feel free to post them below.

Benefits of non-clustered index
A non-clustered index on a table provides fast access to data. If a table has a non-clustered index then the index allows the database engine to locate data quickly without having to scan through the entire table.

How to create a non-clustered index
A non-clustered index can create as like clustered index created in the previous article. Please see the following script which will help you to create a no-clustered index.


-- Adding non-clustered index on a table Employee
CREATE NONCLUSTERED INDEX IX_Employee_EmpLastNameEmpFirstName ON 
dbo.Employee(EmpLastName ASC,EmpFirstName ASC);

CREATE INDEX IX_Employee_EmpFirstName ON dbo.Employee (EmpFirstName ASC);


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