DATEDIFF och GROUP BY

Jag vill visa hur lång tid varje person (log_userID) har varit inloggad totalt. Jag sparar bland annat logLogindate, logLogoutdate och log_userId i tabellen. Detta fungerar inte:

SELECT log_userId, DATEDIFF(minute, logLogindate, logLogoutdate)
FROM login
GROUP BY log_userId

Column login.logLogoutdate is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

DBMS = SQL Server

SELECT log_userId, sum(DATEDIFF(minute, logLogindate, logLogoutdate))
FROM login
GROUP BY log_userId

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essentitia preter non sans multiplicandum

Tackar.