title: Oracle: NOT A GROUP BY EXPRESSION ERROR
body: <p>Below is my query. I am not sure what cause the Not A group by expression error. Any help is much appreciated. =)</p>
  
  <pre><code>SELECT c.courseID, c.courseName, AVG(a.Mark) as Average_Mark
  FROM course c, assessment a
  WHERE c.courseID = a.courseID
  Group by c.courseID, c.courseName
  ORDER BY Mark DESC;
  </code></pre>