title: Union of two tables selecting * but only where there are no duplicates in certain columns
body: <p>If I have two select statements:</p>
  
  <pre><code>select * from x as a
  union 
  select * from y as b
  </code></pre>
  
  <p>but I want to union a to b where b doesn't have the same records as a but only in columns a and b and not * columns, would there be a way to do that?</p>