Difference between Cluster and Non-cluster index?

Answer Posted / indranil

The clustered index is SORTED, and the nonclustered index is
UNSORTED.

For example, we have a pile of birth certificates in City
Hall, and we're going to make an index by birthday (birth
date, to be more exact.)

With a nonclustered index, it's like you have a bunch of
paper lists, each paper with a date, and the people born
under that date. You can quickly determine all people who
were born on August 4, 1986. However the papers themselves
are not sorted, so if you wanted to know all people born in
1986, or August 1986, you'll have to go through all the papers.

With a clustered index, the birth certificates themselves
are sorted by birth date, so searching for people born in
1986, or August 1986, January-June 1981, or any date range
is much faster. Also, it is easier to sort all the people by
birth date (since the data is already sorted).

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