What is difference between unique key with 'not null' constraint and primary key?

Question

I want to know difference between these two key.

When the Unique key with not null constrain in terms of how they are stored in database

and wh

Answer 1

a. Both primary key and unique key enforce uniqueness of the column on which they are defined. But by default, the primary key creates a clustered index on the column, whereas unique key creates a non-clustered index by default. Another major difference is that primary key doesn’t allow NULLs, but unique key allows one NULL only.