The Firebird 5.0 Language Reference | Aggregate Functions Inside Window Specification

Aggregate Functions Inside Window Specification

It is possible to use aggregate functions (but not window functions) inside the OVER clause.In that case, first the aggregate function is applied to determine the windows, and only then the window functions are applied on those windows.

Note	When using aggregate functions inside `OVER`, all columns not used in aggregate functions must be specified in the `GROUP BY` clause of the `SELECT`.

Using an Aggregate Function in a Window Specification

select
  code_employee_group,
  avg(salary) as avg_salary,
  rank() over (order by avg(salary)) as salary_rank
from employee
group by code_employee_group