Another Transact-SQL extension allows you to nest a vector aggregate inside a scalar aggregate. For example, to find the average price of all types of books using a non-nested aggregate, enter:
select avg(price) from titles group by type
--------------- NULL 13.73 11.49 21.48 13.50 15.96 (6 rows affected)
Nesting the average price inside the max function produces the highest average price of a group of books, grouped by type:
select max(avg(price)) from titles group by type
------------- 21.48 (1 row affected)
By definition, the group by clause applies to the innermost aggregate—in this case, avg.
