title: group by clause in oracle10g
body: <p>I am using oracle 10g and wrote a query like this:    </p>
  
  <pre><code>SELECT presenttime,
         employeeid
    FROM mobilelocation
   WHERE presentdate = '31-10-2011'
   GROUP BY employeeid;
  </code></pre>
  
  <p>resulting an error message</p>
  
  <pre><code>ORA-00979: not a GROUP BY expression
  </code></pre>
  
  <p>But upon removing this group clause returining the result without any errors like this,</p>
  
  <pre><code>PRESENTTIME EMPLOYEEID
  23:25   12304
  23:48   12305
  </code></pre>
  
  <p>What is the problem with my group by clause and also suggest me how to use <strong>order by</strong> clause also</p>