一、各个函数介绍

1.OVER(PARTITION BY… ORDER BY…)--开窗函数

1.开窗函数用于为行定义一个窗口（这里的窗口是指运算将要操作的行的集合），它对一组     值进行操作，不需要使用GROUP BY子句对数据进行分组，能够在同一行中同时返回基础     行的列和聚合列。

2. 先把一组数据按照（PARTITION BY）指定的字段分割成各种组，然后组内按照某个字段       排序。（ORDER BY）;

2.OVER()里头的分组以及排序的执行晚于 where 、group by、 早于order by 的执行;

常用组合：

 - row_number() over(partition by ... order by ...)		--分组排序
 - rank() over(partition by ... order by ...)		--分组排序
 - dense_rank() over(partition by ... order by ...)		--分组排序
 - count() over(partition by ... order by ...)		--分组计数
 - max() over(partition by ... order by ...) 		--分组取最大值
 - min() over(partition by    ... order by ...)		--分组取最小值
 - sum() over(partition by ... order by ...)		--分组求和
 - avg() over(partition by ... order by ...)		--分组取平均值
 - first_value() over(partition by ... order by ...)		--取分组第一条
 - last_value() over(partition by ... order by ...)			--取分组最后一条
 - lag() over(partition by ... order by ...)		--取出同一字段的前N行的数据
 - lead() over(partition by ... order by ...)		--取出同一字段的前N行的数据

二、实际使用

        row_number()over()

先说需求：树结构中只获取同一级目录的第一个数据信息，原因是分行与支行信息在同一级，这里我只需要分行的信息，不要支行的信息，具体情况如下图所示：

--目的：只获取同一级目录的第一个数据信息
SELECT JGBH,JGMC
FROM (
    --先根据父id进行分组，并将子id升序排序
  SELECT t.*, ROW_NUMBER() OVER (PARTITION BY SJJG ORDER BY JGBH) rn
  FROM GG_JGBH t
  where SYBZ = 1  --这是我筛选的条件，可更改
  CONNECT BY PRIOR JGBH = SJJG  --SJJG是父id,JGBH是子id
  START WITH SJJG IS NULL --从最高级的父id是空的开始
)
--再只获取每个分组的第一个数据
WHERE rn = 1;

---

         first_value()over()

first_value()over(partition by 列名1，列名2 order by 列名1，列名2)是求一组数据的第一个值

select distinct 
a.date,
a.name,
first_value(date)over(partition by name order by date asc)as `每个人对应最早的date`
,
first_value(date)over(partition by name order by date desc)as `每个人对应最晚的date`
from
(
select '张三'as name,'2021-04-11' as date
union all
select '李四'as name,'2021-04-09' as date
union all
select '赵四'as name,'2021-04-16' as date
union all
select '张三'as name,'2021-03-10'as date
union all
select '李四'as name,'2020-01-01'as date
)a

---

        last_value()over()

last_value()over(partition by 列名1，列名2 order by 列名1，列名2)是求一组数据的最后一个值

last_value()默认是升序，如果限制了是降序，则等同于first_value()升序

select distinct a.date,a.name
,last_value(date)over(partition by name order by date asc)as `每个人对应最晚的date`
from
(
select '张三'as name,'2021-04-11' as date
union all
select '李四'as name,'2021-04-09' as date
union all
select '赵四'as name,'2021-04-16' as date
union all
select '张三'as name,'2021-03-10'as date
union all
select '李四'as name,'2020-01-01'as date
)a

select distinct a.date,a.name
,last_value(date)over(partition by name order by date rows between unbounded preceding and current row)as `(-∞,X)`
,last_value(date)over(partition by name order by date rows between unbounded preceding and unbounded following)as `(-∞,+ ∞)`
,last_value(date)over(partition by name order by date rows between current row and unbounded following)as `(X,+ ∞)`
from
(
select '张三'as name,'2021-04-11' as date
union all
select '李四'as name,'2021-04-09' as date
union all
select '赵四'as name,'2021-04-16' as date
union all
select '张三'as name,'2021-03-10'as date
union all
select '李四'as name,'2020-01-01'as date
)a