title: null value in foreign key
body: <p>I copied the following paragraph from 
  <a href="http://msdn.microsoft.com/en-us/library/ms175464%28v=sql.105%29.aspx" rel="nofollow">http://msdn.microsoft.com/en-us/library/ms175464(v=sql.105).aspx</a></p>
  
  <blockquote>
    <p>A FOREIGN KEY constraint can contain null values; however, if any
    column of a composite FOREIGN KEY constraint contains null values,
    verification of all values that make up the FOREIGN KEY constraint is
    skipped. To make sure that all values of a composite FOREIGN KEY
    constraint are verified, specify NOT NULL on all the participating
    columns.</p>
  </blockquote>
  
  <p>Could someone provide me with a simple example to help me understand the above? What verification the foreign key constraint will do? I think this has something to do with referential integrity.</p>
  
  <p>Thanks a lot. </p>