Difference between Primary Key and Unique Key?

 Updated on October 28, 2014     VIKRAM     1312

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 are unique creates a nonclustered index by default.

Another major difference is that, primary key doesn't allow NULLs, but unique key allows one NULL only.

For more information on Primary Key and Unique Key read here.

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