Sql-server – SQL Server – When to use Clustered vs non-Clustered Index

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I know primary differences between clustered and non clustered indexes and have an understanding of how they actually work. I understand how clustered and non-clustered indexes improve read performance. But one thing I am not sure is that what would be the reasons where I would choose one over the other.

For example: If a table does not have a clustered index, should one create a non-clustered index and whats the benefit of doing

Best Answer

I just want to put in a word of warning: please very carefully pick your clustered index! Every "regular" data table ought to have a clustered index, since having a clustered index does indeed speed up a lot of operations - yes, speed up, even inserts and deletes! But only if you pick a good clustered index.

It's the most replicated data structure in your SQL Server database. The clustering key will be part of each and every non-clustered index on your table, too.

You should use extreme care when picking a clustering key - it should be:

narrow (4 bytes ideal)
unique (it's the "row pointer" after all. If you don't make it unique SQL Server will do it for you in the background, costing you a couple of bytes for each entry times the number of rows and the number of nonclustered indices you have - this can be very costly!)
static (never change - if possible)
ideally ever-increasing so you won't end up with horrible index fragmentation (a GUID is the total opposite of a good clustering key - for that particular reason)
it should be non-nullable and ideally also fixed width - a varchar(250) makes a very poor clustering key

Anything else should really be second and third level of importance behind these points ....

See some of Kimberly Tripp's (The Queen of Indexing) blog posts on the topic - anything she has written in her blog is absolutely invaluable - read it, digest it - live by it!

Syntax:

ALTER TABLE {TABLENAME} 
ADD {COLUMNNAME} {TYPE} {NULL|NOT NULL} 
CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}
WITH VALUES

Example:

ALTER TABLE SomeTable
        ADD SomeCol Bit NULL --Or NOT NULL.
 CONSTRAINT D_SomeTable_SomeCol --When Omitted a Default-Constraint Name is autogenerated.
    DEFAULT (0)--Optional Default-Constraint.
WITH VALUES --Add if Column is Nullable and you want the Default Value for Existing Records.

Notes:

Optional Constraint Name:
If you leave out CONSTRAINT D_SomeTable_SomeCol then SQL Server will autogenerate
a Default-Contraint with a funny Name like: DF__SomeTa__SomeC__4FB7FEF6

Optional With-Values Statement:
The WITH VALUES is only needed when your Column is Nullable
and you want the Default Value used for Existing Records.
If your Column is NOT NULL, then it will automatically use the Default Value
for all Existing Records, whether you specify WITH VALUES or not.

How Inserts work with a Default-Constraint:
If you insert a Record into SomeTable and do not Specify SomeCol's value, then it will Default to 0.
If you insert a Record and Specify SomeCol's value as NULL (and your column allows nulls),
then the Default-Constraint will not be used and NULL will be inserted as the Value.

Notes were based on everyone's great feedback below.
Special Thanks to:
@Yatrix, @WalterStabosz, @YahooSerious, and @StackMan for their Comments.

Best Answer

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Syntax:

Example:

Notes:

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