需求，計算20號部門員工工資出現的次數
使用count() 函式：

SQL> select sal,count(*) time from emp where deptno=20 group by sal;

       SAL   TIME
---------- ----------
      2975      1
      1100      1
      3000      2
       800      1

SQL> 

計算20號部門工資出現的次數，並排出序號
使用dense_rank分析函式

SQL> select sal,dense_rank() over(order
 
 by time desc) as seq
  2  from 
  3  (select sal,count(*) time from emp where deptno=20 group by sal) emp;

       SAL    SEQ
---------- ----------
      3000      1
       800      2
      2975      2
      1100      2

SQL> 

根據序號過濾得到序號為2的結果

SQL> select sal
  2  from
  3  (select sal,dense_rank() over(order
 
 by time desc) as seq
  4  from
  5  (select sal,count(*) time from emp where deptno=20 group by sal) emp) emp
  6  where seq=2;

       SAL
----------
      2975
       800
      1100

SQL> 

利用partition by 子句分別查詢各個部門哪個工資等級的員工多

SQL> select deptno,sal
  2  from
  3  (select deptno,sal,dense_rank() over(partition by
 
 deptno order by time desc) as seq
  4  from
  5  (select deptno,sal,count(*) time from emp group by deptno,sal) emp ) emp 
  6  where seq=1;

    DEPTNO    SAL
---------- ----------
    10   5000
    10   1300
    10   2450
    20   3000
    30   1250

SQL> 

以上就是計算次數出現最多的次數的方法。