使用Oracle的分析函数ROW_NUMBER、DENSE_RANK、RANK

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.