title: Selecting Null in SQLAlchemy
body: <p>I want to do the equivalent of</p>
  
  <pre><code>SELECT * FROM 
  (SELECT foo, bar FROM baz JOIN quux ON baz.id = quux.id
      UNION
  SELECT foo, NULL AS bar FROM baz) 
  GROUP BY (foo, bar) HAVING foo = 'John Doe';
  </code></pre>
  
  <p>using sqlalchemy 0.6, but I can't seem to sneak that <code>NULL</code> in there.  </p>
  
  <p>This roughly what I have so far:</p>
  
  <pre><code>q1 = session.query(Baz.foo, Quux.bar).join(Quux)
  q2 = session.query(Baz.foo, None)
  #                           ^^^^ This breaks!
  </code></pre>