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

[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

[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.