The following query select the most senior with the highest salary from each department. Do you have a better way to do it? Can you use correlated-subquery to achive this? Thank you!

select x.* from
(SELECT deptno, ename, hiredate, sal, ROW_NUMBER()
OVER (PARTITION BY deptno ORDER BY hiredate asc, sal desc) AS senior_rich
FROM emp) x
where senior_rich =1

SQL> /

DEPTNO ENAME HIREDATE SAL SENIOR_RICH
---------- ---------- --------- ---------- -----------
10 CLARK 09-JUN-81 2450 1
20 SMITH 17-DEC-80 800 1
30 ALLEN 20-FEB-81 1600 1

FYI:
SQL> select * from emp;

EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
---------- ---------- --------- ---------- --------- ---------- ---------- ----------
7369 SMITH CLERK 7902 17-DEC-80 800 20
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 CLARK MANAGER 7839 09-JUN-81 2450 10
7788 SCOTT ANALYST 7566 19-APR-87 3000 20
7839 KING PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7876 ADAMS CLERK 7788 23-MAY-87 1100 20
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 MILLER CLERK 7782 23-JAN-82 1300 10

14 rows selected.