[Yii Framework] Using createCommand() cause query problem

$command = Yii::app()->db->createCommand("
                        SELECT * 
                        FROM vote v
                        LEFT JOIN works w
                            ON w.id = v.works_id
                        LEFT JOIN activity a
                            ON a.id = w.activity_id
                        WHERE a.id = $activityId 
                            AND v.vote_date = '$date'
                            AND v.username = '$username'
                    ");
                    $rows = $command->query();$command = Yii::app()->db->