Good job! Now, let's wrap things up. First, here's a summary of what we've covered in this part:

1. Multi-level aggregations require common table expressions (CTEs). The most basic CTE looks like this:

   WITH inner_query_name AS (
     SELECT
       function(column_1) AS agg_column_1
     FROM ...
   )
   SELECT
     function(agg_column_1)
   FROM inner_query_name;
   

2. We can add a GROUP BY clause in the outer query to show multi-level aggregation for multiple business objects:

   WITH inner_query_name AS (
     SELECT
       function(column_1) AS agg_column_1,
       column_2
     FROM ...
   )
   SELECT function(agg_column_1)
   FROM inner_query_name
   GROUP BY column_2;
   

3. We can also use a CASE WHEN construction to introduce our own classifications into multi-level aggregations:

   WITH inner_query_name AS (
     SELECT
       function(column_1) AS agg_column_1, 
       CASE
         WHEN ...
       END AS column_2
     FROM ...
   )
   SELECT function(agg_column_1)
   FROM inner_query_name
   GROUP BY column_2;
   

4. Finally, we can use more than one CTE in a query:

   WITH cte_1 AS (...),
   cte_2 AS (...),
   ...,
   cte_n AS (...)
   SELECT
     ...
   

Let's solve a few problems before we go...