title: Group by: ORA-00937: not a single-group group function
body: <p>Why is this query not working properly?</p>
  
  <pre><code>SELECT e.* 
  FROM enrolled e 
  FULL OUTER JOIN student s ON e.studentid = s.sid 
  WHERE ((e.quarter = 'Fall') OR (e.quarter = 'Spring')) 
  GROUP BY e.studentid 
  HAVING count(e.studentid) == 1;
  </code></pre>
  
  <p>Error that happens:</p>
  
  <blockquote>
    <p>ORA-00937: not a single-group group function 00937. 00000 - "not a single-group group function"   *Cause: *Action: Error at Line: 1 Column: 8 </p>
  </blockquote>
  
  <p>Thanks,</p>