Declare @StartTime	datetime,
	@EndTime	datetime,
	@Total		int

SELECT Client, datepart(hh, YourDate) HourNumber, SUM([NumRows]) VRowCount
INTO #1
FROM YourTable
WHERE YourDate BETWEEN @StartTime and @EndTime
GROUP BY Client, datepart(hh,YourDate)

SET @Total = (SELECT SUM(VRowCount)
		FROM #1)

SELECT Client, HourNumber, VRowCount, @Total TotalRows
FROM #1

SELECT 
    CASE WHEN client IS NOT NULL AND hourSegment IS NULL THEN 'Client Total' ELSE CAST(hourSegment AS VARCHAR) END,
    [No of Rows],
    CASE WHEN client IS NULL AND hourSegment IS NULL THEN 'Grand Total' ELSE client END AS client
FROM (
SELECT LEFT(CONVERT(VARCHAR(30), [datetime], 20), 13) AS hourSegment, client, SUM([no. of rows]) AS [No of Rows]
FROM myTable
GROUP BY LEFT(CONVERT(VARCHAR(30), [datetime], 20), 13), client WITH ROLLUP
)x