# example data
DT <- data.table(Titanic)
Suppose that, for each sex, we want the rows with the highest survival numbers:
DT[Survived == "Yes", .SD[ N == max(N) ], by=Sex]
#    Class    Sex   Age Survived   N
# 1:  Crew   Male Adult      Yes 192
# 2:   1st Female Adult      Yes...