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(customer_id) AS ...
   FROM customers
   WHERE registration_date >= ...
     AND registration_date <  ...;
   

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

   SELECT COUNT(customer_id) AS registration_count
   FROM customers
   WHERE registration_date >= DATE_TRUNC('year', CURRENT_TIMESTAMP);
   

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

   SELECT
     DATE_PART('year', registration_date) AS registration_year,
     COUNT(customer_id) AS registration_count
   FROM customers
   GROUP BY DATE_PART('year', registration_date)
   ORDER BY DATE_PART('year', registration_date);
   

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
     DATE_PART('year', registration_date) AS registration_year,
     channel_name,
     COUNT(*) AS registration_count
   FROM customers cu
   JOIN channels ch
     ON cu.channel_id = ch.id
   GROUP BY DATE_PART('year', registration_date), channel_name
   ORDER BY DATE_PART('year', registration_date);
   

All right! How about a short quiz?