如何仅从左侧 table 获取一条记录，而从右侧 table 获取每条记录

Question

我有两个 table，一个用于某种设备，另一个用于它们的位置（使用这个 table 来记录位置历史记录）。表的架构是：

tbl_Devices:
- DeviceSrNumber（bigint，不为空，pk）
- 其他一些栏目
tbl_DeviceLocation
- SrNumber (bigint, not null, pk, identity)
- DeviceSrNumber（bigint，不为空）
- 位置（varchar(300)，不为空）
- AddDateTime（日期时间，不为空）

我需要的是DeviceSrNumber，最后一个Location，最后一个位置设置AddDateTime

我现在在做什么：

SELECT
    DeviceSrNumber,
    (
        SELECT TOP (1) Location
        FROM tbl_DeviceLocation
        WHERE (DeviceSrNumber = d.DeviceSrNumber)
        ORDER BY AddDateTime DESC
    ) AS 'Location',
    (
        SELECT TOP (1) AddDateTime
        FROM tbl_DeviceLocation
        WHERE (DeviceSrNumber = d.DeviceSrNumber)
        ORDER BY AddDateTime DESC
    ) AS 'AddDateTime '
FROM
    tbl_Devices AS d

但我需要以更简洁的方式使用连接来完成，因为据我所知，子查询效率不高，不建议在大型数据库中使用。

注意：它是一个大型项目的一小部分，它将处理数百万条记录。

Answer 1

你可以从这个开始：

SELECT *
FROM (
    SELECT        
      d.DeviceSrNumber
    , dl.Location    LastLocation
    , dl.AddDateTime LastLocationSetDateTime
    , ROW_NUMBER() OVER(PARTITION BY d.DeviceSrNumber ORDER BY AddDateTime DESC) RN 
    FROM 
        tbl_Devices d
    JOIN tbl_DeviceLocation dl ON dl.DeviceSrNumber = d.DeviceSrNumber 
) D 
WHERE 
    RN = 1

更新

SELECT        
  d.DeviceSrNumber
, MAX(dl.Location)  LastLocation
, MAX(dl.AddDateTime) LastLocationSetDateTime
FROM 
    tbl_Devices d
JOIN tbl_DeviceLocation dl ON dl.DeviceSrNumber = d.DeviceSrNumber 
GROUP BY d.DeviceSrNumber
ORDER BY AddDateTime DESC

Answer 2

这样您的查询会更有效率：

SELECT D.DeviceSrNumber, 
       L.Location, 
       L.AddDateTime 
FROM        tbl_Devices D
OUTER APPLY
   (SELECT TOP (1) DL.Location, DL.AddDateTime
    FROM tbl_DeviceLocation DL
    WHERE D.DeviceSrNumber = DL.DeviceSrNumber
    ORDER BY DL.AddDateTime DESC
   ) L;

Answer 3

试试这个，它会 select 使用最新的 AddDateTime

一个数据

SELECT top(1) D.DeviceSrNumber, L.Location,L.AddDateTime 
FROM tbl_Devices AS D 
JOIN tbl_DeviceLoation AS L ON D.DeviceSrNumber  = L.DeviceSrNumber 
WHERE D.DeviceSrNumber  = L.DeviceSrNumber  order by L.AddDateTime desc

更新

我试过了，它根据最新记录 select 不同的记录。希望这能解决问题

WITH CTE AS 
(
SELECT D.DeviceSrNumber, L.Location,L.AddDateTime , RN= ROW_NUMBER() OVER (PARTITION BY  D.DeviceSrNumber ORDER BY  D.DeviceSrNumber)
FROM  tbl_Devices AS D LEFT JOIN tbl_DeviceLoation AS L ON D.DeviceSrNumber  = L.DeviceSrNumber
WHERE D.DeviceSrNumber  = L.DeviceSrNumber group by  D.DeviceSrNumber, L.Location,L.AddDateTime )
SELECT * FROM CTE where RN=1 order by CTE.AddDateTime desc

如何仅从左侧 table 获取一条记录，而从右侧 table 获取每条记录

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