title: How to do following sql operation using Table.select() method in python sqlobject?
body: <p>Sql Operation:</p>
  <pre><code>    SELECT *,
          CASE
              WHEN quota_unit = 'MB' THEN quota*1024
              ELSE quota
          END AS data_usage
      FROM mapping
      ORDER BY data_usage;
  </code></pre>
  <p>I tried like this,</p>
  <pre><code>query = mapping.select('CASE WHEN mapping.quota_unit = 'MB' THEN 1024 * mapping.quota ELSE mapping.quota END AS data_usage)
  query.orderBy(data_usage)
  res = list(query)
  </code></pre>
  <p>but select method taking that value in braces for Where clause and resultant query becomes</p>
  <pre><code>SELECT * FROM mapping WHERE 'CASE WHEN mapping.quota_unit = MB THEN 1024 * mapping.quota ELSE mapping.quota END AS data_usage'
  </code></pre>
  <p>which is not expected. Can you please me here ?</p>