title: Add columns to sqlalchemy query's SELECT clause
body: <p>I have a base query that I would like to customize with different select clauses depending on a condition.  Essentially</p>
  <pre><code>engine = create_engine(…)
  session = sessionmaker(bind=engine)()
      
  base_query = session.query(Employees.id).\
                       filter(Employees.years_service&gt;5)
  
  if need_name:
      Add a select clause for the name field
  
  if need_birthday:
      Add a select clause for birthday
  </code></pre>
  <p>…except the filter criteria are much more complicated so I’d rather not just create duplicate queries.</p>
  <p>When I want to add additional filter criteria, I can do this kind of decoration by just saying something like:</p>
  <pre><code>if years_service_max_needed:
      base_query.filter(Employees.years_service&lt;10)
  </code></pre>
  <p>but I can’t find a query object method that would allow me to add additional select statements.</p>
  <p>Is there a way to do such a thing?</p>