求助 with as 用法详解

WITH
sum_sales AS 
  select /*+ materialize */ 
    sum(quantity) all_sales from stores
number_stores AS 
  select /*+ materialize */ 
    count(*) nbr_stores from stores
sales_by_store AS
  select /*+ materialize */ 
  store_name, sum(quantity) store_sales from 
  store natural join sales
SELECT
   store_name
FROM
   store,
   sum_sales,
   number_stores,
   sales_by_store
where
   store_sales > (all_sales / nbr_stores)
;

看着有点模糊，求助高手给讲解下，3Q