----SQL254 统计salary的累计和running_total
按照salary的累计和running_total，其中running_total为前N个当前( to_date = ‘9999-01-01’)员工的salary累计和，其他以此类推。
输出顺序：emp_no salary running_total
Demo展示：

emp_no	salary	running_total
10001	88958	88958
10002	72527	161485
10003	43311	204796
10004	74057	278853

表的创建和数据的插入：

drop table if exists  salaries ;
CREATE TABLE salaries (
emp_no int NOT NULL,
salary int NOT NULL,
from_date date NOT NULL,
to_date date NOT NULL,
PRIMARY KEY (emp_no,from_date));
INSERT INTO salaries VALUES(10001,60117,'1986-06-26','1987-06-26');
INSERT INTO salaries VALUES(10001,62102,'1987-06-26','1988-06-25');
INSERT INTO salaries VALUES(10001,66074,'1988-06-25','1989-06-25');
INSERT INTO salaries VALUES(10001,66596,'1989-06-25','1990-06-25');
INSERT INTO salaries VALUES(10001,66961,'1990-06-25','1991-06-25');
INSERT INTO salaries VALUES(10001,71046,'1991-06-25','1992-06-24');
INSERT INTO salaries VALUES(10001,74333,'1992-06-24','1993-06-24');
INSERT INTO salaries VALUES(10001,75286,'1993-06-24','1994-06-24');
INSERT INTO salaries VALUES(10001,75994,'1994-06-24','1995-06-24');
INSERT INTO salaries VALUES(10001,76884,'1995-06-24','1996-06-23');
INSERT INTO salaries VALUES(10001,80013,'1996-06-23','1997-06-23');
INSERT INTO salaries VALUES(10001,81025,'1997-06-23','1998-06-23');
INSERT INTO salaries VALUES(10001,81097,'1998-06-23','1999-06-23');
INSERT INTO salaries VALUES(10001,84917,'1999-06-23','2000-06-22');
INSERT INTO salaries VALUES(10001,85112,'2000-06-22','2001-06-22');
INSERT INTO salaries VALUES(10001,85097,'2001-06-22','2002-06-22');
INSERT INTO salaries VALUES(10001,88958,'2002-06-22','9999-01-01');
INSERT INTO salaries VALUES(10002,72527,'1996-08-03','1997-08-03');
INSERT INTO salaries VALUES(10002,72527,'1997-08-03','1998-08-03');
INSERT INTO salaries VALUES(10002,72527,'1998-08-03','1999-08-03');
INSERT INTO salaries VALUES(10002,72527,'1999-08-03','2000-08-02');
INSERT INTO salaries VALUES(10002,72527,'2000-08-02','2001-08-02');
INSERT INTO salaries VALUES(10002,72527,'2001-08-02','9999-01-01');
INSERT INTO salaries VALUES(10003,40006,'1995-12-03','1996-12-02');
INSERT INTO salaries VALUES(10003,43616,'1996-12-02','1997-12-02');
INSERT INTO salaries VALUES(10003,43466,'1997-12-02','1998-12-02');
INSERT INTO salaries VALUES(10003,43636,'1998-12-02','1999-12-02');
INSERT INTO salaries VALUES(10003,43478,'1999-12-02','2000-12-01');
INSERT INTO salaries VALUES(10003,43699,'2000-12-01','2001-12-01');
INSERT INTO salaries VALUES(10003,43311,'2001-12-01','9999-01-01');

1、采用自联结的方式，逻辑有点难理解，查询如下：

select s1.emp_no, s1.salary, sum(s2.salary) as running_total
from salaries s1
join salaries s2
on s1.emp_no >= s2.emp_no
where s1.to_date='9999-01-01' and s2.to_date='9999-01-01'
group by s1.emp_no,s1.salary
order by emp_no

重点：s1.emp_no>=s2.emp_no，因为这里是sum(s2.salary)，所以结合起来看就是查找小于等于s1.emp_no的s2.salary总和。

2、直接使用窗体函数，sum() over()解决，查询如下：

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