Question

I have the following SQL query:

SELECT      Devices.*
        ,   DevicesActivityData.* 
FROM        Devices
INNER JOIN  DevicesActivityData 
        ON  Devices.ID = DevicesActivityData.DeviceID
WHERE       DevicesActivityData.DeviceDateTime IN (
                SELECT    MAX(DevicesActivityData.DeviceDateTime) AS MaxDate
                FROM      DevicesActivityData                                
                GROUP BY  DevicesActivityData.DeviceID
            )

And I need to complete my code :

var Q = db.Devices.Join(
            db.DevicesActivityDatas, 
            d => d.ID, 
            a => a.DeviceID,
            (d, a) => new { d.ID,d.DeviceName,a.DeviceDateTime }
        );

I have Problem in last part select max

How can I Add this part to my query….

[where DevicesActivityData.DeviceDateTime in ( select max(DevicesActivityData.DeviceDateTime) as MaxDate from DevicesActivityData group by DevicesActivityData.DeviceID )]

Answer 1

First create the query that selects the max dates:

var maxDates = db.DevicesActivityDatas
           .GroupBy(d => d.DeviceID)
           .Select(g => g.Max(t => t.DeviceDateTime));

then do:

 var Q = db.Devices.Join(db.DevicesActivityDatas
                          .Where(m => maxDates.Contains(m.DeviceDateTime)),
                            d => d.ID, a => a.DeviceID,
                           (d, a) => new { d.ID,d.DeviceName,a.DeviceDateTime});

Deferred execution will make both queries run as one.

As a side note I would like to point out that the logic may not be correct. It seems to me that you want a list of devices with their DevicesActivityData that has the most recent date. But the max dates are not related to a device any more, it’s just a list of arbitrary dates. So a device can have more DevicesActivityData records with a date that matches one of the collected dates. I think you need to look for DevicesActivityData records with the max date for that specific device.