聚合主分类，子查询获得子分类-优快云博客

本文提供两个SQL查询案例，一是如何将产品信息表转换为宽格式，二是如何将时间序列数据重塑，以展示每月金额，适用于数据库管理和数据分析人员。

1、原表:
id proid proname
1 1 M
1 2 F
2 1 N
2 2 G
3 1 B
3 2 A
查询后的表:
id pro1 pro2
1 M F
2 N G
3 B A
写出查询语句

构建环境USE [aa]
GO
/****** Object:  Table [dbo].[t]    Script Date: 03/09/2011 10:50:46 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[t](
	[id] [int] NULL,
	[proid] [int] NULL,
	[proname] [nvarchar](50) NULL
) ON [PRIMARY]
GO
INSERT [dbo].[t] ([id], [proid], [proname]) VALUES (1, 1, N'M')
INSERT [dbo].[t] ([id], [proid], [proname]) VALUES (1, 2, N'F')
INSERT [dbo].[t] ([id], [proid], [proname]) VALUES (2, 1, N'N')
INSERT [dbo].[t] ([id], [proid], [proname]) VALUES (2, 2, N'G')
INSERT [dbo].[t] ([id], [proid], [proname]) VALUES (3, 1, N'B')
INSERT [dbo].[t] ([id], [proid], [proname]) VALUES (3, 2, N'A')

答案select 
id,
pro1=(select proname from t where Id = x.Id and proid=1),
pro2=(select proname from t where id = x.Id and proId=2)
from t x 
group by id

2、

面试题：怎么把这样一个表儿
year month amount
1991 1 1.1
1991 2 1.2
1991 3 1.3
1991 4 1.4
1992 1 2.1
1992 2 2.2
1992 3 2.3
1992 4 2.4
查成这样一个结果
year m1 m2 m3 m4
1991 1.1 1.2 1.3 1.4
1992 2.1 2.2 2.3 2.4

环境
/****** Object:  Table [dbo].[t]    Script Date: 03/09/2011 11:09:06 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[t](
	[year] [int] NULL,
	[month] [int] NULL,
	[amount] [float] NULL
) ON [PRIMARY]
GO
INSERT [dbo].[t] ([year], [month], [amount]) VALUES (1991, 1, 1.1)
INSERT [dbo].[t] ([year], [month], [amount]) VALUES (1991, 2, 1.2)
INSERT [dbo].[t] ([year], [month], [amount]) VALUES (1991, 3, 1.3)
INSERT [dbo].[t] ([year], [month], [amount]) VALUES (1991, 4, 1.4)
INSERT [dbo].[t] ([year], [month], [amount]) VALUES (1992, 1, 2.1)
INSERT [dbo].[t] ([year], [month], [amount]) VALUES (1992, 2, 2.2)
INSERT [dbo].[t] ([year], [month], [amount]) VALUES (1992, 3, 2.3)
INSERT [dbo].[t] ([year], [month], [amount]) VALUES (1992, 4, 2.4)

答案select
[year],
m1=(select amount from t where [year]=x.[year] and [month]=1),
m2=(select amount from t where [year]=x.[year] and [month]=2),
m3=(select amount from t where [year]=x.[year] and [month]=3),
m4=(select amount from t where [year]=x.[year] and [month]=4)
from t x
group by [year]