title: Order by an expression in Solr
body: <p>In SQL you can order by an expression like:</p>
  
  <pre><code>SELECT * FROM a
  ORDER BY CASE WHEN a.Category = 20 THEN 1 ELSE 0 DESC
  </code></pre>
  
  <p>so records who have Category = 20 are on top.</p>
  
  <p>Is this possible in Solr?</p>