subtract number from previous days number

[cbiker]

I have an qry that brings up one months worth of flow meter numbers. what i would like to do is subtract the previouse days flow meter number from each days number to get a flow for the day. I tryed Dlookup but couldnt et it to work. any help would be appritiated.

 

[Golom]

Something along the lines of
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Select T.TheDate As [Reading Date],

T.MeterReading As [Reading],

(T.MeterReading -

(Select MAX(MeterReading) From tbl As X
Where X.TheDate < T.TheDate )) As [Flow]

From tbl As T
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