Write a Query to return Running Total of employees Salary within each department.

select 

d.department_name, e.first_name, e.salary, 

sum(e.salary) over (partition by e.department_id order by e.first_name) running_total 

from employees e
left join departments d on e.department_id = d.department_id
order by d.department_name, e.first_name