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 InnerQueryName AS (
     SELECT
       Function(Column1) AS AggColumn1
     FROM ...
   )
   SELECT
     Function(AggColumn1)
   FROM InnerQueryName;
   

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

   WITH InnerQueryName AS (
     SELECT
       Function(Column1) AS AggColumn1,
       Column2
     FROM ...
   )
   SELECT Function(AggColumn1)
   FROM InnerQueryName
   GROUP BY Column2;
   

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

   WITH InnerQueryName AS (
     SELECT
       Function(Column1) AS AggColumn1, 
       CASE
         WHEN ...
       END AS Column2
     FROM ...
   )
   SELECT Function(AggColumn1)
   FROM InnerQueryName
   GROUP BY Column2;
   

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

   WITH Cte1 AS (...),
   Cte2 AS (...),
   ...,
   CteN AS (...)
   SELECT
     ...
   

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