title: Convert NULL to empty string - Conversion failed when converting from a character string to uniqueidentifier
body: <p>Using SQL Server 2005 how do I get the below statement or rather the output as i want it to be.</p>
  
  <pre><code>SELECT Id   'PatientId',
         ISNULL(ParentId,'')  'ParentId'
  FROM Patients
  </code></pre>
  
  <p>ParenId is a <code>uniqueidentifier</code> that allows <code>NULL</code>, but seems that query optimizer tries to also convert <code>''</code> back to <code>uniqueidentifier</code> for the rows where <code>ParentId = NULL</code>.As the title says that's the exact error info the query runner throws at my face!!</p>
  
  <ul>
  <li>How do i get the server to return empty string for <code>ParentId = NULL</code></li>
  </ul>