举例说明：
员工每个月都会有薪水，需要查出每个月的薪水以及累计薪水，可以用下面SQL语句查询：
SELECT MONTH, SALARY, SUM(SALARY) OVER(ORDER BY MONTH ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) SUM_SALARY FROM USER_SALARY
举例说明：
员工每个月都会有薪水，需要查出每个月的薪水以及累计薪水，可以用下面SQL语句查询：
SELECT MONTH, SALARY, SUM(SALARY) OVER(ORDER BY MONTH ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) SUM_SALARY FROM USER_SALARY
