try this code.
echo "select * from tbl_sensor where stype=4 and nid=$nid order by timestamp desc limit 10";
$res= Yii::app()->db->createCommand("select * from tbl_sensor where stype=4 and nid=$nid order by timestamp desc limit 10")->queryAll();
//print_r($res); // will return array of result.
// to check query, print the query executed.
return $res;