title: What is OUTER UNION and why is it partially compatible
body: <p>I am trying to understand how a <code>OUTER UNION</code> works, and why it is only partially compatible. </p>
  
  <p>I am aware this operation was created to take union of tuples from two relations if the relation are not type compatible (which I understand).</p>
  
  <p>Examples of this operation will be great!</p>