We usually use AR in Yii 2.0 it's quick and easy, but for some condition we need use DAO for Database operate, it's direct and faster， follow are some demo code

//Query All
$topics = Yii::$app->db->createCommand('SELECT * FROM yiilib.topics')->queryAll();

//Query One
$topics = Yii::$app->db->createCommand('SELECT * FROM yiilib.topics')->queryOne();

//Get Count
$count = $db->createCommand('SELECT COUNT(*) FROM yiilib.topics')->queryScalar();

//DO topic.commentCount ++
//SQL for update topic.commentCount ++
$sql = 'update yiilib.topic set commentCount = commentCount+1 where topic_id=:topicID';

//create command and run the SQL
\Yii::$app->db->createCommand($sql)->bindValue(':topicID', $this->com_topic_id)->execute();