SqlServer 案例：已有汽車每日行駛里程數據，計算其每日增量

需求說明

某公司某項業務，需要獲得用戶每日行車里程數。已知能獲得該車每日提交的總里程數，如何通過 T-SQL 來獲得其每日增量里程？

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解決方案

0. 首選需要對數據進行編號，利用開窗函數 OVER() 實現隔離（PARTITION）並排序（ORDER BY）車輛數據；
1. 利用公共表表達式（CTE）將上一步虛擬表緩存在本次會話的內存之中；
2. 外部查詢語句查詢 CTE，將所獲記錄與前一條記錄的里程數進行相減，獲得的即為結果。

數據准備

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)

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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

結果截圖：

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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;

結果截圖：