窗口函数使用举例

窗口函数对于一些统计应用情况有非常好的使用效果，本文主要举例使用常用的几种窗口函数。
定义

　　A window function performs a calculation across a set of table rows that are somehow related to the current row.
数据准备

CREATE TABLE empsalary(
  depname varchar,
  empno bigint,
  salary int,
  enroll_date date
);
INSERT INTO empsalary VALUES('develop',10, 5200, '2007/08/01');
INSERT INTO empsalary VALUES('sales', 1, 5000, '2006/10/01');
INSERT INTO empsalary VALUES('personnel', 5, 3500, '2007/12/10');
INSERT INTO empsalary VALUES('sales', 4, 4800, '2007/08/08');
INSERT INTO empsalary VALUES('sales', 6, 5500, '2007/01/02');
INSERT INTO empsalary VALUES('personnel', 2, 3900, '2006/12/23');
INSERT INTO empsalary VALUES('develop', 7, 4200, '2008/01/01');
INSERT INTO empsalary VALUES('develop', 9, 4500, '2008/01/01');
INSERT INTO empsalary VALUES('sales', 3, 4800, '2007/08/01');
INSERT INTO empsalary VALUES('develop', 8, 6000, '2006/10/01');
INSERT INTO empsalary VALUES('develop', 11, 5200, '2007/08/15');
操作1.row_number():返回行号,对比值重复时行号不重复不间断， 即返回 1,2,3,4,5....,不返回 1,2,2,4...

select row_number() over(),* from empsalary limit 2;
 

select row_number() over(),* from empsalary limit 2 offset 2;
 

--按depname分组，salary排序，注意红色记录行号不间断
select row_number() over(partition by depname order by salary),* from empsalary;
2.rank()：返回行号,对比值重复时行号重复并间断， 即返回 1,2,2,4...

select rank() over(partition by depname order by salary),* from empsalary;
3.dense_rank()：返回行号,对比值重复时行号重复但不间断， 即返回 1,2,2,3

select dense_rank() over(partition by depname order by salary),* from empsalary;
4.percent_rank()：从当前开始， 计算在分组中的比例 (行号-1)*(1/(总记录数-1))

select percent_rank() over(partition by depname order by salary),* from empsalary;
5.cume_dist() ：返回行数除以记录数值

select ROUND((cume_dist() over(partition by depname order by salary))::NUMERIC,2) AS cume_dist,* from empsalary;
6.ntile(分组数量):让所有记录按分组数目尽可以的均匀分布

select ntile(3) over(partition by depname order by salary),* from empsalary;
7.lag(value any [, offset integer [, default any ]])：返回偏移量值， offset integer 是偏移值， 正数时前值， 负数时后值， 没有取到值时用 default 代替；所谓正偏移即当前行的值salary按照偏移量offset偏移到下面对应的行，负偏移同。见红色标识

select lag(salary,1,null) over(partition by depname order by enroll_date),* from empsalary;
8.lead(value any [, offset integer [, default any ]])：返回偏移量值， offset integer 是偏移值， 正数时取后值，负数时取前值， 没有取到值时用 default 代替。

select lead(salary,1,2) over(partition by depname order by enroll_date),* from empsalary;
 
9.first_value(value any)返回第一值

select first_value(salary) over(partition by depname order by enroll_date),* from empsalary;
10.last_value(value any)返回最后值

select last_value(salary) over(partition by depname order by enroll_date),* from empsalary;
从返回结果看，似乎有问题，默认情况下， 带了 order by 参数会从分组的起始值开始一直叠加， 直到当前值不同为止，通过修改分组的统计范围就可以实现 order by 参数取最后值.

select last_value(salary) over(partition by depname order by enroll_date range between unbounded preceding and unbounded following),* FROM empsalary;
 11.nth_value(value any, nth integer)：返回窗口框架中的指定值，如nth_value(salary,2),则表示返回字段salary的第二个窗口函数值 

select nth_value(salary,2) over(partition by depname order by enroll_date),* from empsalary;
12.同时调用多个窗口函数可用下面别名简化写法

select sum(salary) over w,avg(salary) over w,* from empsalary window w as (partition by depname order by enroll_date);
与下面写法同：
select sum(salary) over(partition by depname order by enroll_date),avg(salary) over(partition by depname order by enroll_date),* from empsalary;
 