Excellent! Nested CTEs are very handy when you want to compute complex statistics in one query. For example, you can use nested CTEs to compute averages of averages. Take a look:

WITH salesman_sold_items AS (
  SELECT 
    salesman_id,
    city_id,
    SUM(items_sold) AS total_items_sold
  FROM daily_sales d
  JOIN salesman s
    ON d.salesman_id = s.id
  GROUP BY salesman_id, city_id
),

city_average AS (
  SELECT 
    city_id,
    AVG(total_items_sold) AS city_avg
  FROM salesman_sold_items
  GROUP BY city_id
)

SELECT AVG(city_avg)
FROM city_average

The first CTE, salesman_sold_items computes the total number of items sold by each salesman. The second CTE, city_average computes the city-level average. It's a very important performance metric for our company! Finally, the outer query computes the average overall.

Notice the computing the overall average per salesman would render a completely different result.