Great! Knowing which customers are active is important, but it's equally important to understand how much revenue our customers generate. We now want to know the average order value for each weekly registration cohort. Check it out:

SELECT 
  DATEPART(Year, RegistrationDate) AS Year, 
  DATEPART(Week, RegistrationDate) AS Week, 
  AVG(TotalAmount) AS AverageOrderValue
FROM Customers C
JOIN Orders O
  ON C.CustomerID = O.CustomerID
GROUP BY
  DATEPART(Year, RegistrationDate), 
  DATEPART(Week, RegistrationDate)
ORDER BY
  DATEPART(Year, RegistrationDate), 
  DATEPART(Week, RegistrationDate);

In the query above, we need data from two tables: Customers (to get registration dates) and Orders (to get the total amount for each order). Then, we used AVG(TotalAmount) to calculate the average order value.