-Aggregate functions can be used for some group by clause in this case it will give summary information for that specific group.
Example: Display the maximum salary of all departments in company
SELECT department_id as Dept, AVG(salary)as Avg FROM employees GROUP BY department_id
Nested Group functions
Group functions can be nested to a depth of two
Example, display the maximum average salary
SELECT MAX(AVG(Salary)) FROM employees GROUP BY department_id