SqlServer 案例：已有汽车每日行驶里程数据，计算其每日增量

需求说明

某公司某项业务，需要获得用户每日行车里程数。已知能获得该车每日提交的总里程数，如何通过 T-SQL 来获得其每日增量里程？

解决方案

1. 首选需要对数据进行编号，利用开窗函数 OVER() 实现隔离（PARTITION）并排序（ORDER BY）车辆数据；
2. 利用公共表表达式（CTE）将上一步虚拟表缓存在本次会话的内存之中；
3. 外部查询语句查询 CTE，将所获记录与前一条记录的里程数进行相减，获得的即为结果。

数据准备

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)

方法一（限 MSSQL2012+）

WITH ONE 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  dbo.CarData AS C
)
SELECT *
      ,COALESCE(One.Mileage - LAG(One.Mileage) over(PARTITION BY CarId order by One.NodeId),One.Mileage) AS '增量'
FROM ONE

结果截图：

方法二

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 [dbo].[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;

结果截图：