Php 过去24小时ZendFramework中的平均呼叫持续时间

我在ZendFramework中使用MYSQL查询时遇到了一些问题，我想得到过去24小时内的平均呼叫持续时间，并进行如下查询：

SELECT AVG(duration) FROM table1 where calldate > DATE_SUB(CURDATE(), INTERVAL 24 HOUR;

下面是我的ZendFramework中的代码：
型号：

<?php
class Model_Cdr extends Zend_Db_Table_Abstract
{
    protected $_name = 'table1';
    protected $_schema = 'db1';
}

我也试过：

$callModel= new Model_Cdr();
 $avg_24_1 = $callModel->fetchAll("SELECT AVG(duration WHERE calldate > DATE_SUB(CURDATE(), INTERVAL 1 HOUR))");

使用Zend_Db_Table_选择：

使用它：

$callModel = new Model_Cdr();
$select = $callModel->select()
            ->from($callModel, array('average' => 'AVG(duration)'))
           ->where('calldate > DATE_SUB(CURDATE(), INTERVAL 24 HOUR)');

  $row = $callModel->fetchRow($select);
  echo 'Average time: ' . $row->average;

---

谢谢你的评论。消息：SQLSTATE[42S02]：未找到基表或视图。我已经编辑了我的问题。请复习一下

$callModel = new Model_Cdr();
$select = $callModel->select()
                    ->from('table1', array('average' => 'AVG(duration)'))
                    ->where('calldate > DATE_SUB(CURDATE(), INTERVAL 24 HOUR)');

$row = $callModel->fetchRow($select);
echo 'Average time: ' . $row->average;

$callModel = new Model_Cdr();
$select = $callModel->select()
            ->from($callModel, array('average' => 'AVG(duration)'))
           ->where('calldate > DATE_SUB(CURDATE(), INTERVAL 24 HOUR)');

  $row = $callModel->fetchRow($select);
  echo 'Average time: ' . $row->average;