Query problems - Group By and Latest date

This should do the trick:

SELECT [SerialNumber]
,Param1
,MAX([TimeStamp])
FROM [Build Efficiency System].[dbo].[SSIS_SCANNERDATA_TBL]
WHERE Test = 'T120'
GROUP BY SerialNumber, Param1
ORDER BY SerialNumber

You only need to take the max of your timestamp field (and remove it from the group by). The group by all fields is a bit extreme in the previous query (you could use the distinct keyword instead if you had apparent duplicate rows (i.e. replaced the part 3 times in a day).

Is the serial number of the part in the same table - if so presumably it is different for each time that part is replaced. You can use a nested query to get that - however it will run into a problem if there are multiple records with the same date. As it stands at the moment you could not distinuish between them.

If you had records:

SerialNumber Param1 TimeStamp Part_SN
0 Part1 15/03/07 1234
0 Part2 15/03/07 1235
0 Part2 16/03/07 1236
0 Part2 16/03/07 1237
0 Part3 15/03/03 1238

How would you know which of the two Part2 items fitted on 16 Mar to give the serial number of If there are additional fields to determine this then we need to use them

If this does not arise then the query below should serve (substitute correct fieldname for Part_SN):

SELECT B.[SerialNumber]
,B.Param1
,B.[TimeStamp]
,B.Part_SN
FROM (
SELECT [SerialNumber]
,Param1
,MAX([TimeStamp]) AS TimeStamp
FROM [Build Efficiency System].[dbo].[SSIS_SCANNERDATA_TBL]
WHERE Test = 'T120'
GROUP BY SerialNumber, Param1
) A
INNER JOIN [Build Efficiency System].[dbo].[SSIS_SCANNERDATA_TBL] B
ON (A.[SerialNumber] = B.[SerialNumber]) AND
(A.Param1 = B.Param1) AND
(A.[TimeStamp] = B.[TimeStamp]) AND
(B.Test = 'T120')
ORDER BY B.[SerialNumber]

If this is a problem then you will get multiple records in that case - one for each serial number. If the TimeStamp is a datetime which includes the time of the replacement then this will not be an issue (as long as the required serial number is the last record.