Hi,

The column holding the primary key constraint cannot accept null values.whereas column holding the unique constraint can accept null values.

Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index(physical sorting of a database table?s) on the column, where are unique creates a non clustered index (sorted list of references to the table itself.) by default. Another major difference is that, primary key doesn't allow Nulls, but unique key allows one NULL only.

Thanks,