$criteria = new CDbCriteria;
$criteria->select = 't.first_name, t.email'; // select fields which you want in output
$criteria->condition = 't.status = 1';

$data = Users::model()->findAll($criteria);

User::find()->select(['field1', 'field2']);

$active_users_emails = User::model()->findColumn('email', 'active = 1');

**returns array('rajat@gmail.com','rajatsinghal@gmail.com')..**

$active_users_emails=CHtml::listData(User::model()->findAllByAttributes(array('active'=>1)), 'id', 'email');

$criteria = new CDbCriteria;
$criteria->select = 't.first_name, t.email'; // select fields which you want in output
$criteria->condition = 't.status = 1';

$data = Users::model()->findAll($criteria);

foreach($data as $model){
      $rows[] = array_filter($model->attributes);
   }

echo CJSON::encode($rows)

[{"first_name" : "Rohit", "email" : "rohitsuthar@gmail.com"}, {"first_name" : "Rajat", "email" : "rajat@example.com"}]

$emailIds = Yii::app()->db->createCommand(
                 'SELECT email FROM users WHERE is_active = 1'
            )->queryAll();

$emailIds = array_column( $emailIds, 'email' ) );