title: Can Foreign Key be null?
body: <p>In our database project we have a table <code>Sale</code> that has an primary key and two exclusive foreign keys:   <code>Vehicle_ID</code> and <code>Piece_ID</code> . For example if we sell a vehicle we need <code>Vehicle_ID</code> as a foreign key but not <code>Piece_ID</code>. Can we put NULL to <code>Piece_ID</code>, could a foreign key be null?  Or is there a way to do this job?</p>
  
  <p>Thanks.</p>