What is the difference between primary key and a unique key, Database Management System

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

Both primary key and unique key enforces uniqueness of the column on which they are explained. But by default primary key creates a clustered index on the column, where are unique makes a no clustered index by default. Another main difference is that, primary key doesn't permit NULLs, but unique key permits one NULL only.

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