row_number() over (partition by ...

SQL> select a1,a2,
2 rank () over (order by a1) rank,
3 dense_rank () over (order by a1) dense_rank,
4 row_number () over (order by a1) row_number
5 from test;

        A1         A2       RANK DENSE_RANK ROW_NUMBER
---------- ---------- ---------- ---------- ----------
         1          3          1          1          1
         2          4          2          2          2
         3          2          3          3          3
         3          5          3          3          4
         4          2          5          4          5

以下實現分組後按組產生序號。

RANK() OVER (PARTITION BY DepartmentName ORDER BY num)  top20