Microsoft® SQL Server 2012 Unleashed

Nonclustered Indexes

A nonclustered index is a separate index structure, independent of the physical sort order of the data rows in the table. You can have up to 999 nonclustered indexes per table.

A nonclustered index is similar to the index in the back of a book. To find the pages on which a specific subject is discussed, you look up the subject in the index and then go to the pages referenced in the index. This method is efficient as long as the subject is discussed on only a few pages. If the subject is discussed on many pages, or if you want to read about many subjects, it can be more efficient to read the entire book.

A nonclustered index works similarly to the book index. From the index’s perspective, the data rows are randomly spread ...