SQL Server – Primary Key and a Unique Key
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  1. #1
    Join Date
    Jul 2012
    Posts
    2

    SQL Server – Primary Key and a Unique Key

    Hello Everyone,

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

    Kate

  2. #2
    Join Date
    Jul 2012
    Posts
    2
    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

  3. #3
    Join Date
    Nov 2002
    Location
    New Jersey, USA
    Posts
    3,865
    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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