title: field constant using laravel query builder
body: <p>Using laravel/fluent query builder, I'm trying to cause a constant field value to pass through  for a union(ed) selection that is subsequently ordered . I haven't found the recipe to do the following with fluent.  The unions are easy, but how do you get the field constant to work?</p>
  
  <p>Imagine two simple tables (omitted) and a union select:</p>
  
  <pre><code>select field1, field2, 'type1' as field3 from table1
  UNION
  select field1, field2, 'type2' as field3 from table2
  ORDER BY field2
  </code></pre>
  
  <p>The best answer I've come up with so far, is to use a DB::query with a query string I manufacture myself. Laravel/fluent does not seem ready to handle this case, given the test cases I've tried.  Using RAW for a select works great, until you try to order the pair of selected table queries.  </p>
  
  <pre><code>SELECT field1, field2 FROM 
  (
    SELECT fld1A as field1, 'FOO' as field2 from table1 
    UNION ALL
    SELECT fld2A as field1, 'BAR' as field2 from table2 
  )
  temp_table order by somefield
  </code></pre>