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
     DATE_PART('year', registration_date) AS year,
     DATE_PART('week', registration_date) AS week,
     COUNT(*) AS active_customers
   FROM customers
   WHERE CURRENT_DATE - last_order_date < INTERVAL '30' day
   GROUP BY DATE_PART('year', registration_date), DATE_PART('week', registration_date)
   ORDER BY DATE_PART('year', registration_date), DATE_PART('week', registration_date);
   

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

   SELECT
     DATE_PART('year', registration_date) AS year,
     DATE_PART('week', registration_date) AS week,
     AVG(total_amount) AS average_order_value
   FROM customers c
   JOIN orders o
     ON c.customer_id = o.customer_id
   GROUP BY DATE_PART('year', registration_date), DATE_PART('week', registration_date)
   ORDER BY DATE_PART('year', registration_date), DATE_PART('week', registration_date);
   

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

   WITH good_customers AS (
   SELECT
     c.customer_id,
     registration_date,
     AVG(total_amount) AS avg_order_value
   FROM customers c
   JOIN orders o
     ON c.customer_id = o.customer_id
   GROUP BY c.customer_id, registration_date
   HAVING AVG(total_amount) > 1636
   )
   SELECT
     DATE_PART('year', registration_date) AS year,
     DATE_PART('week', registration_date) AS week,
     COUNT(*) AS good_customers
   FROM good_customers
   GROUP BY DATE_PART('year', registration_date), DATE_PART('week', registration_date)
   ORDER BY DATE_PART('year', registration_date), DATE_PART('week', registration_date);
   

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