60、统计salary的累计和running_total

1、题目描述

按照salary的累计和running_total，其中running_total为前两个员工的salary累计和，其他以此类推。 具体结果如下Demo展示。。
CREATE TABLE `salaries` ( `emp_no` int(11) NOT NULL,
`salary` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`from_date`));
输出格式:

emp_no	salary	running_total
10001	88958	88958
10002	72527	161485
10003	43311	204796
10004	74057	278853
10005	94692	373545
10006	43311	416856
10007	88070	504926
10009	95409	600335
10010	94409	694744
10011	25828	720572

2、代码

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；