求各個部門在員工入職時(年份-月份)的該部門的總人數

寫法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;
 

寫法2：

SELECT DEPARTMENT_ID,
   TO_CHAR(HIRE_DATE, 'yyyy/mm') HIRE_YEAR_MONTH,
   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, TO_CHAR(HIRE_DATE, 'yyyy/mm')