数据库SQL实践60：统计salary的累计和running_total

思路：

观察可知running_total是小于或等于当前emp_no工资的总和。

select s.emp_no,s.salary,
(select sum(ns.salary) from salaries ns where ns.emp_no <=s.emp_no and ns.to_date='9999-01-01') as running_total
from salaries s where s.to_date='9999-01-01';