update statement using nested query

Something like this?

UPDATE mytable SET lastLogDate = t.maxDateForUser  
FROM  
(  
    SELECT userid, MAX(logDate) as maxDateForUser  
    FROM mytable  
    GROUP BY userId  
) t  
WHERE mytable.userid = t.userid

You can do this:

UPDATE t
SET t.logDate = t2.LatestDate
FROM YourTable t
INNER JOIN
(
    SELECT userID, MAX(LogDate) LatestDate
    FROM YourTable
    GROUP BY userID
) t2 ON t.userID = t2.userID; 

I don’t know if I understood you correctly. Otherwise be a bit more specific, but from what I get, you should do something along the lines of:

UPDATE `mytable`
SET lastLogDate = (SELECT statement goes here)
WHERE ...