这三个分析函数都可以在各个分组内从1开始排序。
ROW_NUMBER()是没有重复值的，可以利用它实现分页显示。
DENSE_RANK()是连续排序，有两个第二名时仍然跟着第三名。
RANK()是跳跃排序，有两个第二名时接下来就是第四名（同样是在各个分组内）。

SELECT * FROM (
     SELECT deptno, ename, sal, ROW_NUMBER()
     OVER (
       PARTITION BY deptno ORDER BY sal DESC
     ) Top3 FROM emp
)
WHERE Top3 <= 3
/

    DEPTNO ENAME                SAL          TOP3
---------- ---------- ---------- ----------
           10 KING                   5000             1
           10 CLARK               2450             2
           10 MILLER              1300             3

           20 SCOTT               3000             1
           20 FORD                 3000             2
           20 JONES                2975             3

           30 BLAKE               2850             1
           30 ALLEN               1600             2
           30 TURNER            1500             3

9 rows selected.

SELECT * FROM (
     SELECT deptno, ename, sal, DENSE_RANK()
     OVER (
       PARTITION BY deptno ORDER BY sal DESC
     ) TopN FROM emp
)
WHERE TopN <= 3
ORDER BY deptno, sal DESC
/

       DEPTNO ENAME                SAL          TOPN
---------- ---------- ---------- ----------
           10 KING                  5000             1
           10 CLARK               2450             2
           10 MILLER              1300             3   20 SCOTT               3000             1     <--- !
          20 FORD                3000             1     <--- !
           20 JONES               2975             2
           20 ADAMS              1100             3

           30 BLAKE               2850             1
           30 ALLEN               1600             2
           30 TURNER            1500             3

10 rows selected.

SELECT * FROM (
     SELECT deptno, ename, sal, RANK()
     OVER (
       PARTITION BY deptno ORDER BY sal DESC
     ) TopN FROM emp
)
WHERE TopN <= 3
ORDER BY deptno, sal DESC
/

       DEPTNO ENAME                SAL          TOPN
---------- ---------- ---------- ----------
           10 KING                   5000             1
           10 CLARK               2450             2
           10 MILLER              1300             3   20 SCOTT               3000            1     <--- !
          20 FORD                 3000           1     <--- !
           20 JONES               2975            3

           30 BLAKE               2850             1
           30 ALLEN               1600             2
           30 TURNER            1500             3

9 rows selected.