Summan av DateTime-skillnaden i sql (TT.MM)

Gör summeringen före formateringen

SELECT
  ATM,
  CONVERT(VARCHAR(10), SUM(DATEDIFF(Minute, TicketRaisedOn, ClosedOn)) / 60)
  + '.' +
  RIGHT('00' + CONVERT(VARCHAR(2), SUM(DATEDIFF(Minute, TicketRaisedOn, ClosedOn)) % 60), 2)
FROM Ticket
GROUP BY ATM

Sql Fiddle:http://sqlfiddle.com/#!3/eca01/1