Calculate Rate of Change Grouped by Column

[#1]

I, like many others I suppose, are looking at the COVID-19 data (I got mine from https://raw.githubusercontent.com/nytimes/covid-19-data/master/us-states.csv). I would like to plot the daily change in confirmed cases by State. I've been able to work it out by first sorting the data by State, and then calculating the deltas:

If(:state == Lag(:state, 1),
	:cases - Lag(:cases, 1)
)

But is there any way to do this without sorting the data first, using some kind of Group By formulation?

[#2 · Accepted solution]

Something like this might work.....but it will be slow if you have a lot of data

dt = Current Data Table();
curDate = :Date;
curState = :State;
Try( :cases - :cases[(dt < get rows where( :date == (curDate - In Days( 1 )) & :State == curState ))[1]] );

[#3]

With one correction, change from: "dt < get rows", to: "dt << get rows"

[#4]

oops....my error......good catch