20.3. Multiple Aggregation Levels
The rule in SQL is that you cannot nest aggregate functions, such as
SELECT department, MIN(COUNT(items)) -- illegal syntax! FROM Foobar GROUP BY department;
The usual intent of this is to get multiple levels of aggregation; this example probably wanted the smallest count of items within each department. But this makes no sense, because a department (i.e., a group) can have only one count, one minimum, one maximum, one average, and so forth for any expression. The nature of descriptive statistics is that they reduce a group characteristic to a scalar value.
20.3.1. Grouped VIEWs for Multiple Aggregation Levels
Business reports are usually based on a hierarchy of nested levels of aggregation. This type of report ...