Hive窗口函数可以计算一定范围内、一定值域内、或者一段时间内的累积和以及移动平均值等；可以结合聚集函数SUM() 、AVG()等使用；可以结合FIRST_VALUE() 和LAST_VALUE()，返回窗口的第一个和最后一个值。
- 如果只使用partition by子句,未指定order by的话,我们的聚合是分组内的聚合. 
- 使用了order by子句,未使用window子句的情况下,默认从起点到当前行.
window子句： 
- PRECEDING：往前 
- FOLLOWING：往后 
- CURRENT ROW：当前行 
- UNBOUNDED：起点，UNBOUNDED PRECEDING 表示从前面的起点， UNBOUNDED FOLLOWING：表示到后面的终点

1、计算累计和
统计1-12月的累积和，即1月为1月份的值，2月为1、2月份值的和，3月为123月份的和，12月为1-12月份值的和。
关键字解析：
SUM(SUM(amount)) 内部的SUM(amount)为需要累加的值；
ORDER BY month 按月份对查询读取的记录进行排序，就是窗口范围内的排序；
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW 定义起点和终点，UNBOUNDED PRECEDING 为起点，表明从第一行开始, CURRENT ROW为默认值，就是这一句等价于：
ROWS UNBOUNDED PRECEDING
PRECEDING：在前 N 行的意思。
FOLLOWING：在后 N 行的意思。

1.1、计算所有月份的累计和

select pt_month,sum(amount) pay_amount,sum(sum(amount))over(order by pt_month) cumulative_amount

from data_chushou_pay_info

where pt_month between '2017-01' and '2017-11' and state=0

group by pt_month;

select pt_month,sum(amount) pay_amount,sum(sum(amount))over(order by pt_month ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) cumulative_amount

from data_chushou_pay_info

where pt_month between '2017-01' and '2017-11' and state=0

group by pt_month;

1.2、计算前3个月和本月共4个月的累积和

select pt_month,sum(amount) pay_amount,sum(sum(amount))over(order by pt_month ROWS BETWEEN 3 PRECEDING AND CURRENT ROW) cumulative_amount

from data_chushou_pay_info

where pt_month between '2017-01' and '2017-11' and state=0

group by pt_month;

select pt_month,sum(amount) pay_amount,sum(sum(amount))over(order by pt_month ROWS 3 PRECEDING) cumulative_amount

from data_chushou_pay_info

where pt_month between '2017-01' and '2017-11' and state=0

group by pt_month;

1.3、计算前1月后1月和本月共3个月的累积和

select pt_month,sum(amount) pay_amount,sum(sum(amount))over(order by pt_month ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) cumulative_amount

from data_chushou_pay_info

where pt_month between '2017-01' and '2017-11' and state=0

group by pt_month;

2、计算平均值
2.1、计算前1月后1月和本月共3个月各月总值的平均值

select pt_month,sum(amount) pay_amount,avg(sum(amount))over(order by pt_month ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) average_amount

from data_chushou_pay_info

where pt_month between '2017-01' and '2017-11' and state=0

group by pt_month;

2.2、计算前3个月和本月共4个月各月总值的平均值

select pt_month,sum(amount) pay_amount,avg(sum(amount))over(order by pt_month ROWS BETWEEN 3 PRECEDING AND CURRENT ROW) cumulative_amount

from data_chushou_pay_info

where pt_month between '2017-01' and '2017-11' and state=0

group by pt_month;

3、计算窗体第一条和最后一条的值

select pt_month,sum(amount) pay_amount,first_value(sum(amount))over(order by pt_month ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) first_amount,last_value(sum(amount))over(order by pt_month ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) last_amount

from data_chushou_pay_info

where pt_month between '2017-01' and '2017-11' and state=0

group by pt_month;