知识点

1. 本题有两种写法一种直接使用窗口函数，一种使用子查询
2. 函数名(column) over(选项),所有聚合函数都可以作为窗口函数
3. 子查询就是sum使用的表员工编号小于等于外层表员工编号

代码

窗口函数
select emp_no, salary, 
sum(salary) over (order by emp_no)  as running_total
from salaries
where to_date = '9999-01-01'

子查询
select s1.emp_no, s1.salary, 
(select sum(s2.salary)
 from salaries as s2
 where s2.emp_no <= s1.emp_no
 and s2.to_date = '9999-01-01'
) as running_total
from salaries as s1
where s1.to_date = '9999-01-01'
order by s1.emp_no