title: What is difference between unique key with 'not null' constraint and primary key?
body: <p>I want to know difference between these two key.</p>
  
  <p>When the Unique key with <code>not null</code> constrain in terms of how they are stored in database </p>
  
  <p>and what difference are there when we making <code>Select,Insert,Update, Delete</code> operation for these keys.</p>