Well done! It's time to wrap things up. Let's make a quick summary of what we've learned:

1. Conversion rate is the count of customers that performed a specific desired action divided by the count of all customers.
2. To calculate the global lifetime conversion (all customers who ever placed an order), use:

   SELECT
     ROUND(COUNT(FirstOrderId) / CAST(COUNT(*) AS FLOAT), 2) AS ConversionRate
   FROM Customers;
   

3. To show conversion rates in weekly registration cohorts, use the DATEPART() function:

   SELECT
     DATEPART(Year, RegistrationDate) AS Year,
     DATEPART(Week, RegistrationDate) AS Week,
     ROUND(COUNT(FirstOrderId) * 100 / COUNT(*), 2) AS ConversionRate
   FROM Customers
   GROUP BY 
     DATEPART(Year, RegistrationDate),
     DATEPART(Week, RegistrationDate)
   ORDER BY
     DATEPART(Year, RegistrationDate),
     DATEPART(Week, RegistrationDate);
   

4. To calculate the time from registration to first order, use:

   SELECT
     CustomerID,
     DATEDIFF(Day, RegistrationDate, FirstOrderDate) AS DaysToFirstOrder
   FROM Customers;
   

5. To create a conversion chart, use multiple COUNT(CASE WHEN...) instances:

   SELECT
     DATEPART(Year, RegistrationDate) AS Year,
     DATEPART(Week, RegistrationDate) AS Week,
     ...
     COUNT(CASE WHEN DATEDIFF(Day, RegistrationDate, FirstOrderDate) < 7 THEN CustomerID END) AS OneWeek,
     COUNT(CASE WHEN DATEDIFF(Day, RegistrationDate, FirstOrderDate) >= 7 AND DATEDIFF(Day, RegistrationDate, FirstOrderDate) < 14 THEN CustomerID END) AS TwoWeeks,
     ...
   FROM Customers
   GROUP BY 
     DATEPART(Year, RegistrationDate),
     DATEPART(Week, RegistrationDate)
   ORDER BY
     DATEPART(Year, RegistrationDate),
     DATEPART(Week, RegistrationDate);
   

All right! How about a short quiz?