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SQL :: What's the difference between a primary key and a unique key?

posted Aug 16, 2012, 5:50 AM by Ashish Jain   [ updated Aug 16, 2012, 5:54 AM ]
Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary, key creates a clustered index on the column, where as unique key creates a nonclustered index by default. Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.