SQL Function

Are you looking for the below query,

SELECT FiveMinutes, Avg(Val) as Val FROM (
        SELECT 
        DATEADD(minute,(DATEDIFF(minute,DayStart,[DateTime])/5)*5,DayStart) AS
        FiveMinutes, Val FROM (SELECT *,
        DATEADD(day,DATEDIFF(day,0,[DateTime]),0) as DayStart
        FROM table1) as A
        ) X
GROUP BY FiveMinutes

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Regards, RSingh