A non-clustered index is merely a pointer to the row containing data.
It corresponds to the index of a textbook: You find the keyword and then go to the specified page.
Non-clustered indexes do not affect the order of the rows in the table.
When you make a request for a particular column, SQL Server searches through the index, finds the actual records and goes directly there.
This is not as efficient as searching with a clustered index but is still far superior to looking through the table record by record in search of a value.
A non-clustered index is especially helpful when the query is searching for an exact value.
You can create multiple non-clustered indexes for different columns or combinations
of columns and use them in different queries.