by using your given example I tried

select convert(varchar(20), Reading_Date), sum(Fuel_Level) as fuel from dbo.Levels

where convert(date,Reading_Date)=convert(date,getdate())

group by Convert (varchar(20),Reading_Date)

order by convert(datetime, convert(varchar(20),Reading_Date))

it displaying like this :

Sep 6 2012 12:00AM179152.80

Sep 6 2012 12:15AM179509.60

Sep 6 2012 12:30AM180141.50

Sep 6 2012 12:45AM180243.30

Sep 6 2012 1:00AM180227.70

Sep 6 2012 1:15AM180196.60

Sep 6 2012 1:30AM180175.20

I want in output to sum up fuel_level from 6 2012 12:00AM to 6 2012 12:45AM and dispaly date as sep 6 2012

-- expected output

date hour fuel_level

2012-09-06 0 50

2012-09-06 1 500

- - -

- - -

2012-09-06 24 2530

cheers

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