我想我至少让它工作感谢这个Return all data from 1st Table, and only 1 data from 2nd Table if exist
我不确定这个解决方案的效率有多高（或者我应该说是占用资源）。

SELECT
        r.id as id,
        reports.Timestamp as reportTimestamp,
        reports.Latitude as reportLat,
        reports.Longitude as reportLng,
        FROM Requests r
        LEFT JOIN 
        
        (SELECT 
          c.* 
        FROM
          PositionReports c 
          LEFT JOIN PositionReports d 
            ON c.RequestId = d.RequestId 
            AND c.Timestamp < d.Timestamp 
        WHERE d.RequestId IS NULL) reports
        
        ON r.id = reports.RequestId

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没有关于资源消耗的声明-另一种方法，使用RANK窗口函数：

SELECT
  r.id as id,
  reports.Timestamp as reportTimestamp,
  reports.Latitude as reportLat,
  reports.Longitude as reportLng
FROM RequestsTable r
LEFT JOIN (
  SELECT
    *,
    RANK() OVER (PARTITION BY RequestId ORDER BY Timestamp DESC) as rnk
  FROM PositionReports
  ) reports
  on reports.RequestId = r.id
  and reports.rnk = 1
;

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在行动中看到它：DB Fiddle的数据。
如果需要调整/进一步详细说明，请予以说明。

是否有一种方法来检索所有这样的报告表中没有一个条目以及？
您的代码的问题在于您正在使用where子句进行过滤-因此这将驱逐left join中不匹配的记录。您可以将过滤移动到join的on子句，如下所示：

select
    r.id mission_id,
    pr.Timestamp as reportTimestamp,
    pr.Latitude as reportLat,
    pr.Longitude as reportLng
from Requests r
left join PositionReports pr 
    on  pr.RequestId = r.id
    and pr.id = ( select max(pr1.id) from PositionReports pr1 where pr1.RequestId = r.Id)

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现在我们可以看到，第一个连接条件似乎是多余的，因为第二个条件覆盖了它;这应该足够了：

select
    r.id mission_id,
    pr.Timestamp as reportTimestamp,
    pr.Latitude as reportLat,
    pr.Longitude as reportLng
from Requests r
left join PositionReports pr 
    on pr.id = ( select max(pr1.id) from PositionReports pr1 where pr1.RequestId = r.Id)

型
最后：如果你正在运行MySQL 8.0.13或更高版本，这看起来是一个非常好的横向连接点：

select r.id mission_id,
    pr.Timestamp as reportTimestamp,
    pr.Latitude as reportLat,
    pr.Longitude as reportLng
from Requests r
left join lateral (
    select pr.*
    from PositionReports pr 
    where pr.RequestId = r.Id
    order by id desc limit 1
) pr on 1

型
子查询与外部查询关联，并直接返回您要查找的记录;对于PositionReports(RequestId, id desc)上的索引，这应该是一个有效的解决方案。