# Database Discussions > Microsoft SQL Server 2005 >  SQL Server  Primary Key and a Unique Key

## katty.jonh

Hello Everyone,

I would like to know Whats the Difference between a Primary Key and a Unique Key?

Kate

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## nimolccd

Hi Kate,

Both primary key and unique key enforce uniqueness of the column on which they are defined. But by default, the primary key creates a clustered index on the column, whereas unique key creates a non-clustered index by default. Another major difference is that primary key doesnt allow NULLs, but unique key allows one NULL only.

Nimol
Programmer
Corbin Creative Databases, LLC 
http://www.corbincreative.com

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## skhanal

You can have only one primary key in a table but can have multiple unique keys, which by the way violates 3rd normal form.

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