How to get First Row of Each group in a Table

WITH FirstEmployeeOf_EachDepartment
AS
(
SELECT E.Employee_ID , E.Name,E.Age , E.Department_ID,

ROW_NUMBER () OVER (PARTITION BY E.NAME ORDER BY E.Employee_ID Desc) AS RN

 FROM Employee E)

SELECT F.Employee_ID,F.Name,f.Age,f.Department_ID FROM FirstEmployeeOf_EachDepartment F
 Where f.RN = 1

WITH FirstEmployeeOf_EachDepartment
AS
(
SELECT E.Employee_ID , E.Name,E.Age , E.Department_ID,

ROW_NUMBER () OVER (PARTITION BY E.NAME ORDER BY E.Employee_ID Desc) AS RN

 FROM Employee E)

SELECT F.Employee_ID,F.Name,f.Age,f.Department_ID FROM FirstEmployeeOf_EachDepartment F
 Where f.RN = 1