What is an index in SQL?

Question

What is an index in SQL? Can you explain or reference to understand clearly?

Where should I use an index?

Answer 1

A clustered index is like the contents of a phone book. You can open the book at 'Hilditch, David' and find all the information for all of the 'Hilditch's right next to each other. Here the keys for the clustered index are (lastname, firstname).

This makes clustered indexes great for retrieving lots of data based on range based queries since all the data is located next to each other.

Since the clustered index is actually related to how the data is stored, there is only one of them possible per table (although you can cheat to simulate multiple clustered indexes).

A non-clustered index is different in that you can have many of them and they then point at the data in the clustered index. You could have e.g. a non-clustered index at the back of a phone book which is keyed on (town, address)

Imagine if you had to search through the phone book for all the people who live in 'London' - with only the clustered index you would have to search every single item in the phone book since the key on the clustered index is on (lastname, firstname) and as a result the people living in London are scattered randomly throughout the index.

If you have a non-clustered index on (town) then these queries can be performed much more quickly.

Hope that helps!