求各个部门在员工入职时(年份-月份)的该部门的总人数

写法1：

SELECT DISTINCT HIRE_YEAR_MONTH,DEPARTMENT_ID,COUNT_ FROM ( SELECT DEPARTMENT_ID,TO_CHAR(HIRE_DATE,'yyyy/mm') HIRE_YEAR_MONTH,COUNT(*) OVER(PARTITION BY DEPARTMENT_ID ORDER BY TO_CHAR(HIRE_DATE,'yyyy/mm')) AS COUNT_ FROM HR.EMPLOYEES) ORDER BY DEPARTMENT_ID,HIRE_YEAR_MONTH;

写法2：

SELECT 
DEPARTMENT_ID,COUNT(*),SUM(COUNT(*)) OVER(PARTITION BY DEPARTMENT_ID ORDER BY TO_CHAR(HIRE_DATE,'yyyy/mm')) 
AS COUNT_ 
FROM HR.EMPLOYEES
GROUP BY DEPARTMENT_ID,'yyyy/mm')