pull records at 15 min intervals

[LadyDee]

Given your test data in a table called Test_1 With Fields Called [Timestamp], NValue, TxtCode, the following does what you want.

SELECT 
    [Timestamp],  NValue, TxtCode
    FROM
    (
    SELECT 
    ROW_NUMBER() OVER(Partition By Interval_15  Order BY [TimeStamp] ) as RowNum,*

    FROM (
        SELECT 
        DATEADD(MINUTE, FLOOR(DATEDIFF(MINUTE, '2020-01-01', [TIMESTAMP]) / CAST(15 AS DECIMAL)) * 15, '2020-01-01') AS Interval_15,* 
        From 
        Test_1
        ) as q1

    ) as q2
  Where RowNum = 1

Results

Timestamp               NValue          TxtCode
2021-01-11 11:30:09.000    0.9014763960    A
2021-01-11 11:45:09.000    0.8264825220    B
2021-01-11 12:00:09.000    0.0000000000    C
2021-01-11 12:15:09.000    0.0000000000    A

The Row_Number is a T_SQL window function. If you just run the centre query (q1) on it's own, then the second one (q2) with the first one it will become obvious how they work.

You could probably mash it all into one massive Select statement but it's easier to see what is happening with the sub queries.

Thank you Minty - this works!

[LadyDee]

About SQL windowing functions: the included zip file contains my basic SQL training - you'll find examples of windowing functions on page 33

Thank you very much NoellaG. I will review it closely over the weekend

[Minty]

Thank you Minty - this works!

Glad it helped.
Note, that the hard coded "start dates" ('2020-01-01') will need to be before any of your existing or possible record dates, so you might want to put them further back to something like '1900-01-01'

The windowing functions in SQL Server are awesome and fast, and make some things soooo much easier than they would be in Access.