SQL Server Primary Key and a Unique Key
I would like to know Whats the Difference between a Primary Key and a Unique Key?
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 doesnt allow NULLs, but unique key allows one NULL only.
Corbin Creative Databases, LLC
You can have only one primary key in a table but can have multiple unique keys, which by the way violates 3rd normal form.