DATEDIFF in HH:MM:SS format

Question

I need to calculate the total length in terms of Hours, Minutes, Seconds, and the average length, given some data with start time and end time.

For example the resul

Answer 1

I slightly modified Avinash's answer as it may end with error if difference is too big. If you need only HH:mm:ss it is sufficient to distinguish at seconds level ony like this:

SELECT CONVERT(time, 
  DATEADD(s, 
    DATEDIFF(s, 
      '2018-01-07 09:53:00', 
      '2018-01-07 11:53:01'), 
     CAST('1900-01-01 00:00:00.0000000' as datetime2)
   )
)