SELECT 
DATEDIFF(ss,TranIn,TranOut) AS SECONDS,
COUNT(*) AS CNT
FROM #tmp
GROUP BY DATEDIFF(ss,TranIn,Tranout) 

/* result
SECONDS	CNT
0	1
1	1
4	1
5	1
9	1
12	1
15	1
*/

;with CTE AS
(  -- get the elapsed time in seconds for each transaction, with a count by # of seconds
select sec = DateDiff(second, TranIn, TranOut), qty = count(*)
  from #tmp
 group by DateDiff(second, TranIn, TranOut)
)
, Tally AS  -- if you already have a tally table, you can omit it here and use it below
(  -- build a tally table of numbers between 1 and 10
select Number from master.dbo.spt_values where [Type] = 'P' and number between 1 and 10
)
-- now bring them all together
select [Second] = Tally.Number,
       [Quantity of Transactions] = case when CTE.sec is null then 0 else CTE.qty end,
       [Description] = convert(varchar(2), Tally.Number) + 
                       ' second' + 
                       case when Tally.Number = 1 then '' else 's' end + 
                       ' -- ' + 
                       convert(varchar(10), case when CTE.sec is null then 0 else CTE.qty end) + 
                       ' transactions'
  from Tally
    LEFT OUTER JOIN CTE ON Tally.Number = CTE.sec 
 where Tally.Number between 1 and 10  -- this line is only needed if using your own tally table.