Perfect! It's time to wrap things up.

1. To find the number of customers active within the last 30 days in weekly registration cohorts, use:

   SELECT 
     DATEPART(Year, RegistrationDate) AS Year, 
     DATEPART(Week, RegistrationDate) AS Week, 
     COUNT(*) AS ActiveCustomers
   FROM Customers
   WHERE DATEDIFF(Day, LastOrderDate, GETDATE()) < 30
   GROUP BY
     DATEPART(Year, RegistrationDate), 
     DATEPART(Week, RegistrationDate)
   ORDER BY
     DATEPART(Year, RegistrationDate), 
     DATEPART(Week, RegistrationDate);
   

2. To find the average order value in weekly registration cohorts, use:

   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);
   

3. To find the number of "good customers" in weekly registration cohorts, use:

   WITH GoodCustomers AS (
   SELECT
     C.CustomerID,
     RegistrationDate,
     AVG(TotalAmount) AS AvgOrderValue
   FROM Customers C
   JOIN Orders O
     ON C.CustomerID = O.CustomerID
   GROUP BY C.CustomerID, RegistrationDate
   HAVING AVG(TotalAmount) > 1636
   )
   SELECT 
     DATEPART(Year, RegistrationDate) AS Year, 
     DATEPART(Week, RegistrationDate) AS Week,
     COUNT(*) AS GoodCustomers
   FROM GoodCustomers
   GROUP BY
     DATEPART(Year, RegistrationDate), 
     DATEPART(Week, RegistrationDate)
   ORDER BY
     DATEPART(Year, RegistrationDate), 
     DATEPART(Week, RegistrationDate);
   

How about a quick quiz before we move on to the next part?