require(data.table) ## 1.9.5
X = data.table(x=c(1,1,1,2,2), y=1:5, key="x")
Y = data.table(x=2:5, y=letters[1:4], key="x")
X[Y]
#    x  y i.y
#1: 2  4   a
#2: 2  5   a
#3: 3 NA   b
#4: 4 NA   c
#5: 5 NA   d
X[Y, .N]
# [1] 2
X[Y, .N, nomatch=0L]
# [1] 2

X[Y, sum(y)] # [1] NA - correct result
X[Y, sum(y, na.rm=TRUE)] # correct result