Question

我有两个表，一个表用于某种设备，另一个表用于其位置（使用此表记录位置历史记录）。表的架构为：

tbl_Devices：
- DeviceSrNumber（bigint，不为null，pk）
- 其他一些列
tbl_DeviceLocation
- SrNumber（bigint，不为null，pk，身份）
- DeviceSrNumber（bigint，不为null）
- 位置（varchar（300），不为null）
- AddDateTime（DateTime，不为null）

我需要的是DeviceSrNumber，最后一个Location，最后一个位置集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

但是我需要以一种更简洁的方式进行连接，因为据我所知，子查询效率不高，因此不建议在大型数据库中使用。

注意：这只是大型项目的一小部分，它将处理数百万条记录。

Answer 1

您可以从这里开始：

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

您的查询将以这种方式更加高效：

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

尝试此操作，它将使用最新的AddDateTime选择一个数据

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

我尝试过此操作，它根据最新记录选择了不同的记录。希望能解决

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

如何只从左表获得一个记录，而从右表获得每个记录

3 个答案: