What are the differences between primary keys and unique indexes in SQL Server ?

differences between primary keys and unique indexesI was recently asked, what are the differences between primary keys and unique indexes? Well, they are very similar but here are the differences.

Only one primary key is allowed on a table but multiple unique indexes can be added up to the maximum allowed number of indexes for the table (SQL Server = 250 (1 x clustered, 249 x non clustered) and SQL 2008 and SQL 2012 = 1000 (1 x clustered, 999 x non clustered)).

Primary keys cannot contain nullable columns but unique indexes can. Note, that only one NULL is allowed. If the index is created across multiple columns, each combination of values and NULL’s must be unique.

By default, unless you specify otherwise in the create statement and providing that a clustered index does not already exists, the primary key is created as a clustered index. Unique indexes however are created by default as non clustered indexes unless you specify otherwise and providing that a clustered index does not already exist.

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About Andy Hayes

Andy Hayes is a DBA working with SQL Server since version 7.0. He has a wonderful wife and two beautiful children. He loves database technology, playing cricket, and blogging. He is passionate about sharing his experiences as a DBA and learning more to further his understanding and knowledge. You can follow me on Twitter, check out my Facebook page or follow me on

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