Question

What I’m trying to do is find the last ID key column for each SerialNumber column within a table.

Here’s the sample tables, truncated to just the columns I need to answer this question.

Devices Table
SerialNumber
12345
45678
67890

History Table
ID | SerialNumber
1  | 12345
2  | 45678
3  | 67890
4  | 12345
5  | 67890

My expected output would be:

ID | SerialNumber
2  | 45678
4  | 12345
5  | 67890

Here’s the SQL I’m trying to convert from:

SELECT max(ID) as ID, SerialNumber
FROM History
WHERE SerialNumber in
 (SELECT Distinct SerialNumber
 FROM Devices
 WHERE DeviceType = 0)
GROUP BY SerialNumber)

Here’s the best working linq query I’ve got:

var query= from ListOfIDs in History
           where (from distinctSerialNubers in Devices
           where distinctSerialNubers.DeviceType == 0
           select distinctSerialNubers.SerialNumber).Contains(ListOfIDs.SerialNumber)
           select new {ListOfIDs.ID, ListOfIDs.SerialNumber};

Using .Max() will return the last ID in the History table. Trying to group ListOfIDs by ListOfIDs.SerialNumber will separate all ids by SerialNumber, but I can’t seem to find a way to get into the groups to run .Max().

Edit:
Thank you for the answers. This combination of the two answers works well for me.

var query= from historyEntry in History
where (from s in Devices where s.DeviceType==0 select s.SerialNumber).Contains(historyEntry.SerialNumber)
group historyEntry by historyEntry.SerialNumber into historyGroup
select historyGroup.Max(entry => entry.ID);

Thank you both, I’ve been trying to figure this one out all day.

Answer 1

Haven’t compiled/tested it but this should be what you’re looking for:

from historyEntry in db.History
join device in db.Devices on history.SerialNumber equals device.SerialNumber
where device.DeviceType == 0
group historyEntry by historyEntry.SerialNumber into historyGroup
select new { ID = historyGroup.Max(entry => entry.ID), SerialNumber = historyGroup.Key }