Perfect! It's time to wrap things up. What have we learned in this section?

1. To count registrations during a given period of time, use:

   SELECT COUNT(CustomerID) AS ...
   FROM customers
   WHERE RegistrationDate >= ...
     AND RegistrationDate <  ...;
   

2. To count registrations from the current period (e.g., the current year), use:

   SELECT COUNT(CustomerID) AS RegistrationCount
   FROM Customers
   WHERE RegistrationDate >= DATEADD(year, DATEDIFF(year, 0, GETDATE()), 0);
   

3. To show a report of registration counts for all years, use:

   SELECT
     DATEPART(year, RegistrationDate) AS RegistrationYear,
     COUNT(CustomerID) AS RegistrationCount
   FROM Customers
   GROUP BY DATEPART(year, RegistrationDate)
   ORDER BY DATEPART(year, RegistrationDate);
   

4. A customer cohort is a group of customers that share a common characteristic over a certain time period. To show annual registration cohorts per channel, use:

   SELECT
     DATEPART(year, RegistrationDate) AS RegistrationYear,
     ChannelName,
     COUNT(*) As RegistrationCount
   FROM Customers Cu
   JOIN Channels Ch
     ON Cu.ChannelId = Ch.Id
   GROUP BY DATEPART(year, RegistrationDate), ChannelId, ChannelName
   ORDER BY DATEPART(year, RegistrationDate);
   

All right! How about a short quiz?