this article takes the initial EMP table of SCOTT users as a reference. The code can be used directly.
The statement to query the structure of the EMP table is as follows, code 1:

DESC EMP;

The structure of EMP table is as follows: [result 1]:

SQL> DESC EMP;
 //Is the name empty? Type
 ----------------------------------------------------------------- -------- --------------------------------------------
 EMPNO                                                             NOT NULL NUMBER(4)
 ENAME                                                                      VARCHAR2(10)
 JOB                                                                        VARCHAR2(9)
 MGR                                                                        NUMBER(4)
 HIREDATE                                                                   DATE
 SAL                                                                        NUMBER(7,2)
 COMM                                                                       NUMBER(7,2)
 DEPTNO                                                                     NUMBER(2)

                      .
                   

SELECT EMPNO,ENAME,SAL,DEPTNO
FROM EMP
ORDER BY SAL DESC;

The sorting results are as follows, [result 2]:

SQL> SELECT EMPNO,ENAME,SAL,DEPTNO
  2  FROM EMP
  3  ORDER BY SAL DESC;

     EMPNO ENAME             SAL     DEPTNO
---------- ---------- ---------- ----------
      7839 KING             5000         10
      7902 FORD             3000         20
      7788 SCOTT            3000         20
      7566 JONES            2975         20
      7698 BLAKE            2850         30
      7782 CLARK            2450         10
      7499 ALLEN            1600         30
      7844 TURNER           1500         30
      7934 MILLER           1300         10
      7521 WARD             1250         30
      7654 MARTIN           1250         30
      7876 ADAMS            1100         20
      7900 JAMES             950         30
      7369 SMITH             800         20

//14 rows selected.

Now I want to find out the highest paid employees in each department. What should I do? Go on...

Method 1 (nested query & max() function & self join)

SELECT EMPNO,ENAME,SAL,DEPTNO
FROM EMP E1
WHERE
    SAL = (
        SELECT MAX(SAL)
        FROM EMP E2
        WHERE E1.DEPTNO=E2.DEPTNO
    )
ORDER BY DEPTNO;

The query result is as follows, result 3:

SQL> SELECT EMPNO,ENAME,SAL,DEPTNO
  2  FROM EMP E1
  3  WHERE
  4      SAL = (
  5          SELECT MAX(SAL)
  6          FROM EMP E2
  7          WHERE E1.DEPTNO=E2.DEPTNO
  8      )
  9  ORDER BY DEPTNO;

     EMPNO ENAME             SAL     DEPTNO
---------- ---------- ---------- ----------
      7839 KING             5000         10
      7788 SCOTT            3000         20
      7902 FORD             3000         20
      7698 BLAKE            2850         30

Method 2 (nested query & rank() function & list partition)

SELECT EMPNO,ENAME,SAL,DEPTNO
FROM (
    SELECT
        RANK() OVER(PARTITION BY DEPTNO ORDER BY SAL DESC) R,
        EMPNO,ENAME,SAL,DEPTNO
    FROM EMP
)
WHERE R=1
ORDER BY DEPTNO;

The query result is as follows, result 4:

SQL> SELECT EMPNO,ENAME,SAL,DEPTNO
  2  FROM (
  3      SELECT
  4          RANK() OVER(PARTITION BY DEPTNO ORDER BY SAL DESC) R,
  5          EMPNO,ENAME,SAL,DEPTNO
  6      FROM EMP
  7  )
  8  WHERE R=1
  9  ORDER BY DEPTNO;

     EMPNO ENAME             SAL     DEPTNO
---------- ---------- ---------- ----------
      7839 KING             5000         10
      7788 SCOTT            3000         20
      7902 FORD             3000         20
      7698 BLAKE            2850         30

Remarks:

• Environment: Windows 10
• Software: Oracle 11g R2.