答案：

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

难点在于：running_total为前N个当前( to_date = '9999-01-01')员工的salary累计

(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