Cumulative sum by group in sqldf?

Question

I have a data frame with 3 variables: place, time, and value (P, T, X). I want to create a fourth variable which will be the cumulative sum of X. Normally I like to do group

Answer 1

I hope i understood what you want:

library(plyr)
ddply(df, .(P,T), summarize, cumsum(X))

does this help you?