title: SQL case statement logic issue
body: <p>I've written the following case statement to bring out a list of results for a particular subject.</p>
  
  <pre><code>SELECT upn,
         Min(CASE name
               WHEN 'English' THEN
                 CASE
                   WHEN result IS NULL THEN 'U'
                   ELSE result
                 END
             END) English
  FROM   subject
  GROUP  BY upn 
  </code></pre>
  
  <p>Where the result is NULL I'd like to replace this with a U else bring through the results, but the above doesn't replace the NULLS. It works if I remove the <code>ELSE result</code> portion, but this obviously ends up displaying NULLS and Us. Something isn't quite right with my logic. Can anyone spot what's wrong?</p>