Question

I would like to take the average of the time difference from Table1 below. The values are not consecutive and occasionally the time value is repeated, so I need to 1) sort by time, 2) discard non-unique values, 3) perform the time difference (in milliseconds), then 4) average the resulting time difference values. Further I’d like to 5) limit the datediff operation to a chosen time range, such as
WHERE _TimeStamp >= ‘20091220 11:59:56.1’ AND _TimeStamp <= _TimeStamp >= ‘20091220 11:59:56.8’. I am pretty stumped how to put this all together!

Table1:
_TimeStamp
2009-12-20 11:59:56.0
2009-12-20 11:59:56.5
2009-12-20 11:59:56.3
2009-12-20 11:59:56.4
2009-12-20 11:59:56.4
2009-12-20 11:59:56.9

Answer 1

Here’s one that works and is not ugly:

;WITH Time_CTE AS
(
    SELECT
        MIN(_Timestamp) AS dt,
        ROW_NUMBER() OVER (ORDER BY MIN(_Timestamp)) AS RowNum
    FROM Table1
    GROUP BY _Timestamp
)
SELECT
    t1.dt AS StartDate,
    t2.dt AS EndDate,
    DATEDIFF(MS, t1.dt, t2.dt) AS Elapsed
FROM Time_CTE t1
INNER JOIN Time_CTE t2
ON t2.RowNum = t1.RowNum + 1

Will give you the following output from your example:

StartDate               | EndDate                 | Elapsed
------------------------+-------------------------+--------
2009-12-20 11:59:56.000 | 2009-12-20 11:59:56.300 | 300
2009-12-20 11:59:56.300 | 2009-12-20 11:59:56.400 | 100
2009-12-20 11:59:56.400 | 2009-12-20 11:59:56.500 | 100
2009-12-20 11:59:56.500 | 2009-12-20 11:59:56.900 | 400

Edit: If you want to restrict time ranges then just add WHERE _Timestamp BETWEEN @StartDate AND @EndDate before the GROUP BY line.

Edit2: And if you want the average, then change that final SELECT t1.dt, ... statement to:

SELECT AVG(DATEDIFF(MS, t1.dt, t2.dt))
FROM Time_CTE t1 ... (same as above)