Questions

1. How do aggregate functions evaluate NULL?
   The NULL value is almost ignored in aggregate functions. NULL is not a minimal nor maximal value. The only exception to this rule is the COUNT(*) aggregate function, because it works with whole records.
2. When executing an aggregate query, the following error occurred: Column '#src.SubcategoryName' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause. What does this mean?
   This error means that we forgot to add a non-constant column to the GROUP BY clause from the SELECT clause.
3. What are GROUPING SETS used for?
   When writing aggregate queries, we sometimes need to have more combinations of grouping criteria. GROUPING SETS are used to define several combinations of grouping in aggregate queries.
4. How is the frame defined when the ranking function is used?
   When...