-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