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SQL Server issue with unique keys


The distinction between a unique key and a primary key, as well as between a unique key constraint and a main key constraint in SQL Server, confuses me much.

Can my table have more than one primary key defined? Once more, I'm confused on how to add a unique key in my table and what it will actually be used for. Second, I discovered this quote on the internet.

A UNIQUE constraint is defined on a PRIMARY KEY constraint by default.

It should be noted that each table can have an unlimited number of UNIQUE constraints, but only one PRIMARY KEY constraint.

Does this imply that if I establish a primary key, the unique key associated with it is immediately defined, allowing for a single insertion of a null value? 

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