Improve company productivity with a Business Account.Sign Up

x
?
Solved

Primary Key - Clustered Index - SQL Server

Posted on 2010-08-24
4
Medium Priority
?
620 Views
Last Modified: 2012-05-10
Hi,

I thought that when you create a primary key, clustered index is automatically created. Am i correct or wrong ? But here is what i have got:

When you create a PRIMARY KEY constraint, a unique clustered index on the column or columns is automatically created if a clustered index on the table does not already exist and you do not specify a unique nonclustered index.

Please explain me on detail manner whether i am correct or wrong. Provide simple examples to understand it better.

I did not follow this statement: and you do not specify a unique nonclustered index => Please explain in detail with example.

Thanks
0
Comment
Question by:milani_lucie
4 Comments
 
LVL 58

Accepted Solution

by:
cyberkiwi earned 668 total points
ID: 33517678
-- specifically stated as nonclustered
CREATE TABLE TEST1 ( ID INT NOT NULL PRIMARY KEY NONCLUSTERED )

-- not stated, so it will default to clustered
CREATE TABLE TEST2 ( ID INT NOT NULL PRIMARY KEY )


CREATE TABLE TEST3 ( ID INT NOT NULL)

-- not stated, so it will default to clustered
Alter table Test3 add constraint pk_test1 primary key(ID)
     or
-- specifically stated as nonclustered
Alter table Test3 add constraint pk_test1 primary key nonclustered(ID)


CREATE TABLE TEST4 ( ID INT NOT NULL, X INT NOT NULL UNIQUE CLUSTERED)

-- the following will not be clustered because Test4 already has a clustered index
Alter table Test4 add constraint pk_test1 primary key(ID)

-- the following won't even work
Alter table Test4 add constraint pk_test1 primary key clustered(ID)
0
 
LVL 7

Assisted Solution

by:rashmi_vaghela
rashmi_vaghela earned 664 total points
ID: 33517956
0
 
LVL 5

Expert Comment

by:ThakurVinay
ID: 33518811
When you create a primary key unique cluster index automatically creates but it is not necessory you can create a primary key with out unique cluster index.

if you try to create primary key with GUI you will understand it better.

HTH
Vinay
0
 
LVL 22

Assisted Solution

by:dportas
dportas earned 668 total points
ID: 33524987
No a clustered index is not ncessarily created automatically. If you don't specify the type of index for the PRIMARY KEY column(s) then a clustered index will be created unless a clustered index is specified for some other column(s).
If you want to be sure, then it's best to specify whether you want a CLUSTERED or NONCLUSTERED index. The primary key isn't necessarily the best choice for a clustered index.
0

Featured Post

Free Tool: Path Explorer

An intuitive utility to help find the CSS path to UI elements on a webpage. These paths are used frequently in a variety of front-end development and QA automation tasks.

One of a set of tools we're offering as a way of saying thank you for being a part of the community.

Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

In the first part of this tutorial we will cover the prerequisites for installing SQL Server vNext on Linux.
One of the most important things in an application is the query performance. This article intends to give you good tips to improve the performance of your queries.
This videos aims to give the viewer a basic demonstration of how a user can query current session information by using the SYS_CONTEXT function
Viewers will learn how to use the SELECT statement in SQL and will be exposed to the many uses the SELECT statement has.

606 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question