title: Can a foreign key be NULL and/or duplicate?
body: <p>Please clarify two things for me:</p>
  
  <ol>
  <li>Can a Foreign key be NULL?</li>
  <li>Can a Foreign key be duplicate?</li>
  </ol>
  
  <p>As fair as I know, <code>NULL</code> shouldn't be used in foreign keys, but in some application of mine I'm able to input <code>NULL</code> in both Oracle and SQL Server, and I don't know why.</p>