Row_Number() Over Partition By

This function can come in very handy. As shown in this example, Row_Number numbers each row within a department in order of the hiredate.

Select     Row_Number()
                Over
                (Partition By Deptno
                     Order By Hiredate)
           As
       Ord,
       -------------------------------
       EmpNo,
       Ename,
       Job,
       HireDate,
       Deptno
  From Emp;

This then allows you to generate a report of the first person to be hired by each department.

Select *
  From
      (
      Select     Row_Number() 
                      Over 
                   (Partition By Deptno
                        Order By Hiredate) 
                 As
             Ord,
             -------------------------------
             EmpNo, 
             Ename,
             Job,
             HireDate,
             Deptno
        From Emp
      )
  Where Ord = 1
/

See Oracle documentation for further detail:
Partition By clause