group by 잘 몰라서 그냥 넘어감 . 다시 해볼것

https://thebook.io/006977/ch04/02/05/04-02/

모두의 SQL: RANK, DENSE_RANK, ROW_NUMBER : 데이터 값에 순위 매기기 - 2

SELECT A.employee_id,
       A.department_id,
       B.department_name,
       salary,

--       RANK()       OVER(PARTITION BY A.department_id ORDER BY salary DESC) RANK_급여,--
--       DENSE_RANK() OVER(PARTITION BY A.department_id ORDER BY salary DESC) DENSE_RANK_급여,--
--       ROW_NUMBER() OVER(PARTITION BY A.department_id ORDER BY salary DESC) ROW_NUMBER_급여

FROM employees A, departments B
WHERE A.department_id = B.department_id
ORDER BY B.department_id, A.salary DESC;