Question

It should return some fields from the SystemTable and the LoadStatus column of the latest record in the ProcessHistory table. The relationship is 1 to many:

SELECT ST.[SystemDetailID], ST.[SystemName], LH.LatestLoadStatus
FROM [SystemTable] AS ST
LEFT OUTER JOIN 
    (
     SELECT LHInner.LoadStatus AS LatestLoadStatus, LHInner.SystemDetailID FROM [dbo].[LoadHistory] AS LHInner
        WHERE LHInner.LoadHistoryID in 
        (
            SELECT LatestLoadHisotoryID FROM 
            (
                SELECT MAX(LoadHistoryID) as LatestLoadHisotoryID, SystemDetailID FROM [dbo].[LoadHistory]
                GROUP BY SystemDetailID
            ) l
        )   
    ) AS LH ON ST.SystemDetailID = LH.SystemDetailID

Thanks,

Answer 1

This is a greatest-n-per-group query.

One Approach

SELECT ST.[SystemDetailID],
       ST.[SystemName],
       LH.LatestLoadStatus
FROM   [SystemTable] AS ST
       OUTER APPLY (SELECT TOP 1 *
                    FROM   [dbo].[LoadHistory] LH
                    WHERE  ST.SystemDetailID = LH.SystemDetailID
                    ORDER  BY LoadHistoryID DESC) LH  

You can also use row_number

WITH LH
     AS (SELECT *,
                ROW_NUMBER() OVER (PARTITION BY SystemDetailID 
                                       ORDER BY LoadHistoryID DESC) RN
         FROM   [dbo].[LoadHistory])
SELECT ST.[SystemDetailID],
       ST.[SystemName],
       LH.LatestLoadStatus
FROM   [SystemTable] AS ST
       LEFT JOIN LH
         ON LH.SystemDetailID = ST.SystemDetailID
            AND LH.RN = 1