目录

• 窗口和开窗函数
• 表旋转

1.窗口和开窗函数

• 窗口
  用户指定的一组行（分区）。
• OVER子句

  OVER ( [PARTITION BY value_exp, ...[ n ] ] 
  	<ORDER BY_Clause> )
  

• 开窗函数
  1）排名开窗函数，ROW_NUMBER()、RANK()、DENSE_RANK()、NTILE()
  2）聚合开窗函数AVG()、SUM()等

IF DB_ID('tt') IS NOT NULL
	DROP DATABASE tt
GO
CREATE DATABASE tt
GO

CREATE TABLE tt.dbo.voc
(
	item varchar(20),
	color varchar(20),
	quantity int
)
INSERT INTO tt.dbo.voc
VALUES
	('Chair', 'blue', 1), ('Chair', 'blue', 2),
	('Chair', 'red', 3), ('yizi', 'red', 1),
	('yizi', 'blue', 6)
GO

-- 先分区，按分区排序，再按分区内行按order by指定列排序
SELECT *
	, ROW_NUMBER() OVER(PARTITION BY color ORDER BY color) AS row_num  -- 编号
	, RANK() OVER(ORDER BY quantity) AS rank_  -- 排名
	, DENSE_RANK() OVER(ORDER BY quantity) AS dense_rank_  -- 排名
	, NTILE(3) OVER(PARTITION BY color ORDER BY color) AS ntile_3  -- 分组，均分
FROM tt.dbo.voc

-- + CASE_Clause
SELECT *
	, CASE NTILE(2) OVER(ORDER BY color DESC)
		WHEN 1 THEN '组1'
		WHEN 2 THEN '组2'
	 END AS '分组'
FROM tt.dbo.voc

-- ntile(2) 均分
delete from tt.dbo.voc
where item='yizi'
	AND color='blue'
GO
insert into tt.dbo.voc
values
	('yizi', 'yellow', 1), ('yizi', 'yellow', 1)
GO
select *
	, NTILE(2) OVER(PARTITION BY color ORDER BY color) AS ntile_3  -- 分组
from tt.dbo.voc  -- 见图，color中元素也被均分

-- 开窗聚合函数
select * from tt.dbo.voc
go
select item
	, quantity
	, AVG(quantity) OVER(PARTITION BY item) AS class_avg
	, quantity - AVG(quantity) OVER(PARTITION BY item) AS class_diff
from tt.dbo.voc

注：利用OVER子句实现多种聚合计算，且语句中存在多个OVER时不影响查询效率。

-- 简化多种聚合计算

select item
	, quantity
	, AVG(quantity) OVER(PARTITION BY item) AS class_avg
	, quantity - AVG(quantity) OVER(PARTITION BY item) AS class_diff
	, AVG(quantity) OVER() AS all_avg
	, quantity - AVG(quantity) OVER() AS vs_all_diff	
	, SUM(quantity) OVER() AS all_sum
from tt.dbo.voc

2.表旋转

• PIVOT
  行 -> 列

  SELECT <非旋转列>
  	[第1个旋转列] AS <别名>
  	...
  	[第n个旋转列]
  FROM
  	(<SELECT生成源数据>) AS <源别名>
  PIVOT
  (
  	<聚合函数>(<被聚合列>)
  FOR
  [<包含将被转换为列标头的值的列>]
  	IN ([第1个旋转后的列], ... [第n个旋转后的列])
  ) AS <旋转表别名>
  [可选<ORDER BY_Clause>]
  

  select * from tt.dbo.voc
  go
  select item
  	, [blue]
  	, [yellow]
  	, [red]
  from tt.dbo.voc
  pivot
  (
  	sum(quantity)
  	for color in ([blue], [yellow], [red]) 
  ) AS pivot_color
  

• UNPIVOT
  列 -> 行

select item,
	[blue], [yellow], [red]
  into tt.dbo.pivot_voc
from tt.dbo.voc
pivot
(
	sum(quantity)
	for color in ([blue], [yellow], [red]) 
	) AS pivot_color
go
select * from tt.dbo.voc
go
select * from tt.dbo.pivot_voc
go
select item, color, quantity
from tt.dbo.pivot_voc
unpivot
(
	quantity 
	for color in ([blue], [red], [yellow])
) AS voc