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 SalesmanSoldItems AS (
  SELECT
    SalesmanId,
    CityId,
    SUM(ItemsSold) AS TotalItemsSold
  FROM DailySales d
  INNER JOIN Salesman S
    ON D.SalesmanId = S.Id
  GROUP BY SalesmanId,
    CityId
),
CityAverage AS (
  SELECT
    CityId,
    AVG(TotalItemsSold) AS CityAvg
  FROM SalesmanSoldItems
  GROUP BY CityId
)

SELECT
  AVG(CityAvg) AS AvgCityAvg
FROM CityAverage

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

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