title: Blobs are not allowed in this expression
body: <p>I face the following problem:</p>
  
  <blockquote>
    <p><em>-615: Blobs are not allowed in this expression.</em></p>
  </blockquote>
  
  <pre><code>SELECT ser
        ,task_code
        ,trans_serial
        ,trans_year
        ,CASE when nvl(length(signed_content), 0) &lt;&gt; '' THEN  signed_content  ELSE  attach_content  END AS signed_content
        ,CASE WHEN nvl(length(signed_content), 0) &lt;&gt; '' THEN 'signed_content' ELSE 'attach_content' END AS comes_from
  FROM attach_detail
  WHERE serial = 5 AND task_code = 88 AND trans_year = 2012 AND trans_serial = 23728
  </code></pre>
  
  <hr>
  
  <p>Note: <code>signed_content</code>, <code>attach_content</code> are of type <code>BYTE</code>; the other columns are of type <code>INT</code>.</p>