SQLServer的学习场景（关于row_number()和COALESCE()的使用）

--使用Sql语句，统计出每辆汽车每天行驶的里程数(不是总里程)

以下为脚本

CREATE TABLE [dbo].[CarData](
[CarID] [int] NULL,
[Mileage] [int] NULL,
[M_year] [int] NULL,
[M_Month] [int] NULL,
[M_Day] [int] NULL
) ON [PRIMARY]
GO
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (1, 10, 2015, 1, 1)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (1, 15, 2015, 1, 2)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (1, 15, 2015, 1, 5)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (1, 20, 2015, 1, 6)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (1, 26, 2015, 1, 9)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (1, 30, 2015, 1, 10)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (1, 35, 2015, 1, 11)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (2, 20, 2015, 1, 5)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (2, 22, 2015, 1, 8)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (2, 40, 2015, 1, 10)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (2, 45, 2015, 1, 11)
INSERT [dbo].[CarData] ([CarID], [Mileage], [M_year], [M_Month], [M_Day]) VALUES (3, 50, 2015, 1, 11)

解题思路：
--关键点：如何上一条记录，有rowNum之类的函数没？有! row_number()
--按照carID排序后，进行rowNum
SELECT ROW_NUMBER() OVER (order by c.carID ) as rowNum,c.* FROM CarData c

--按照carID分组后
SELECT c.CarID, ROW_NUMBER() OVER (PARTITION by c.CarID ORDER BY c.M_year,c.M_Month,c.M_Day) as RowNum ,c.Mileage FROM CarData c

--博友答案
WITH TWO AS(
SELECT ROW_NUMBER() OVER(PARTITION BY CarId ORDER BY CarId, M_Year, M_Month, M_Day) AS NodeId
,C.CarId
,C.Mileage
,C.M_Year
,C.M_Month
,C.M_Day
FROM cardata AS C
)
SELECT A.*
, A.Mileage - COALESCE(B.NextMileage, 0) AS '增量'
FROM TWO AS A
OUTER APPLY (SELECT Mileage AS NextMileage FROM TWO AS B WHERE B.NodeId = A.NodeId - 1 AND B.CarId = A.CarId ) AS B;

---补充知识 函数COALESCE
--说明 至少应有一个参数为 NULL 类型。,返回第一个非NUll类型的值
select COALESCE(null,null);
SELECT COALESCE(NULL,NULL,GETDATE());
SELECT COALESCE(NULL,0);