I've got this table: CREATE TABLE #Data1 ( [Market] VARCHAR(100) NOT NULL, [Operator] VARCHAR(100) NOT NULL, [Date] DATETIME NOT NULL, [Measure] VARCHAR(100) NOT NULL, [Amount] NUMERIC(36,10) NOT NULL, --new calculated fields [DailyAvg_30days] NUMERIC(38,6) NULL DEFAULT 0 ) I've populated all the fields apart from DailyAvg_30days. This field needs to show the total for the preceding 30 days e.g. 1. if Date for a particular record is 2nd Dec then it will be the total for the period 3rd Nov - 2nd Dec inclusive. 2. if Date for a particular record is 1st Dec then it will be the total for the period 2nd Nov - 1st Dec inclusive. My attempt to try to find these totals before updating the table is as follows: SELECT a.[Market], a.[Operator], a.[Date], a.[Measure], a.[Amount], [DailyAvg_30days] = SUM(b.[Amount]) FROM #Data1 a INNER JOIN #Data1 b ON a.[Market] = b.[Market] AND a.[Operator] = b.[Operator] AND a.[Measure] = b.[Measure] AND a.[Date] >= b.[Date]-30 AND a.[Date] <= b.[Date] GROUP BY a.[Market], a.[Operator], a.[Date], a.[Measure], a.[Amount] ORDER BY 1,2,4,3 Is this a valid approach or do I need to approach this from a different angle?