I have done this by this way
$sql = new Sql($this->adapter);
$select = $sql->select();
$select->from('mock_paper');
$select->columns(array(
'is_section'
));
$select->where(array('exam_id = ?' => $exam_id,'level_id = ?' => $level_id))->limit(1);
$sqlstring = $sql->buildSqlString($select);
echo $sqlstring;
die();