Yii2, get an average value from timestamps

I have a basic table with orders, which have a field thats called created_at which is in timestamp format and I want to get the avarage on how many orders have been created per day.

Found a other similar qestion about something like mine question whichI have posted below in the hope that everybody understand what im trying to do.

//not working
$q = new yiidbQuery();
$q->from('shopping_cart');
$total = $q->average('DATEDIFF(`created_at`, CURRENT_DATE())'); 

Answer

Method average in $q->average('DATEDIFF('created_at', CURRENT_DATE())'); just adds AVG(DATEDIFF('created_at', CURRENT_DATE())) to SQL command.

As Jiri Semmler said, what you want to do is about SQL not Yii.

All you need to do is find the count of records for the period you are interested in and divide it by the number of days of that period.

It can be something like

// Define period
$timeFrom = '2018-11-30';
$timeTo = '2018-12-02';

// Number of days for the period
$days = date_diff(date_create($timeFrom), date_create($timeTo))->format("%a");

// Query count of records between dates
$q = new yiidbQuery();
$total = $q->from('order')
  ->where(['between', 'created_at', $timeFrom, $timeTo])
  ->count();

// Find average records per day
$average = $total / $days;

If you have Order model class:

// Query count of records between dates
$total = Order::find()
  ->where(['between', 'created_at', $timeFrom, $timeTo])
  ->count();