SQL Server2008- Calculation of Avg. timing
===

I have a query

    select *,right(convert(varchar, 
    cast(JOB_DONE_time-JOB_send_time as datetime), 121), 12) [TimeTaken]
    from dbo.insert_data
    order by job_id desc  

and result of this query is like

         JOB_ID               JOB_DONE_TIME           JOB_SEND_TIME           TimeTaken
        -------------------- ----------------------- ----------------------- ------------
        001-99900            2011-12-28 13:36:57.860 2011-12-28 13:36:57.593 00:00:00.267
        001-99800            2011-12-28 13:36:57.843 2011-12-28 13:36:57.593 00:00:00.250
        001-99700            2011-12-28 13:36:57.687 2011-12-28 13:36:57.547 00:00:00.140
        001-99600            2011-12-28 13:36:57.593 2011-12-28 13:36:57.547 00:00:00.047
        001-99500            2011-12-28 13:36:57.563 2011-12-28 13:36:57.437 00:00:00.127
        001-99400            2011-12-28 13:36:57.547 2011-12-28 13:36:57.437 00:00:00.110
        001-99300            2011-12-28 13:36:57.483 2011-12-28 13:36:57.377 00:00:00.107
        001-99200            2011-12-28 13:36:57.437 2011-12-28 13:36:57.377 00:00:00.060
        001-99100            2011-12-28 13:36:57.407 2011-12-28 13:36:57.187 00:00:00.220
        001-99000            2011-12-28 13:36:57.360 2011-12-28 13:36:57.187 00:00:00.173
        001-9900             2011-12-28 13:32:22.657 2011-12-28 13:32:22.500 00:00:00.127



and by this query i have got `TimeTaken` column. Now I want to calculate addition and avg. of column `timetaken`. I  have tried with `SUM()` function but it gives me error.

How can i calculate it???