Perform a semi-join with data.table

Question

How do I perform a semi-join with data.table? A semi-join is like an inner join except that it only returns the columns of X (not also those of Y), and does not repeat the r

Answer 1

I'm confused with all the not-joins above, isn't what you want simply:

unique(x[y, .SD])
#   x y
#1: 1 a

If x can have duplicate keys, then you can unique y instead:

## Creating an example data.table 'a' three-times-repeated first row 
x <- data.table(x = c(1,1,1,2), y = c("a", "a", "a", "b"))
setkey(x, x)
y <- data.table(x = c(1, 1), z = 10:11)
setkey(y, x)

x[eval(unique(y, by = key(y))), .SD] # data.table >= 1.9.8 requires by=key(y)
#    x y
# 1: 1 a
# 2: 1 a
# 3: 1 a