get rows of unique values by group

Question

I have a data.table and want to pick those lines of the data.table where some values of a variable x are unique relative to another variable y

It\'s possible to get

Answer 1

The simpler data.table solution is to grab the first element of each group

> dt[, head(.SD, 1), by=.(y, x)]
   y x z
1: a 1 1
2: a 2 2
3: b 3 4
4: b 2 5
5: b 1 6