title: I'm having problem with SUM() in MySQL
body: <p>I want to fetch the <strong>BalanceAmt</strong> of a particular Customer attended by a particular attender. The criteria is that a customer may buy more than one items. Each bought items has been recorded individually. At some cases '<strong><em>attender</em></strong>' value would be NULL. </p>
  
  <p>Here's my query:</p>
  
  <pre><code>SELECT Customer, AttendedBy, SUM(BalanceAmt) FROM billing GROUP BY Customer 
       HAVING AttendedBy='attender' AND Customer='CustomerName'
  </code></pre>
  
  <p>My problem is I'm getting the value summed up not attended by the '<strong><em>attender</em></strong>' (i.e., including NULL value). 
  Help me to do it.. </p>
  
  <p>Thanx in advance..</p>