Well done! It's time to wrap things up. Let's do 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 ratio of all customers who ever placed an order to all registered customers, use:

   SELECT
     ROUND(COUNT(first_order_id) / CAST(COUNT(*) AS float), 2) AS conversion_rate
   FROM customers;
   

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

   SELECT
     DATE_PART('year', registration_date) AS year,
     DATE_PART('week', registration_date) AS week,
     ROUND(COUNT(first_order_id) * 100.0 / COUNT(*), 2) AS conversion_rate
   FROM 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);
   

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

   SELECT
     customer_id,
     first_order_date - registration_date AS days_to_first_order
   FROM customers;
   

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

   SELECT
     DATE_PART('year', registration_date) AS year,
     DATE_PART('week', registration_date) AS week,
     ...
     COUNT(CASE
       WHEN first_order_date - registration_date <  INTERVAL '7' day
       THEN customer_id
       END) AS one_week,
     COUNT(CASE
       WHEN first_order_date - registration_date >= INTERVAL '7' day
        AND first_order_date - registration_date <  INTERVAL '14' day
       THEN customer_id
       END) AS two_weeks,
     ...
   FROM 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);
   

All right! How about a short quiz?