What does < stand for in data.table joins with on=

Question

Joining the data tables:

X <- data.table(A = 1:4, B = c(1,1,1,1)) 
#    A B
# 1: 1 1
# 2: 2 1
# 3: 3 1
# 4: 4 1

Y <- data.table(A = 4)
#    A
# 1: 4
<         


        

Answer 1

When doing a non-equi join like X[Y, on = .(A < A)] data.table returns the A-column from Y (the i-data.table).

To get the desired result, you could do:

X[Y, on = .(A < A), .(A = x.A, B)]

which gives:

   A B
1: 1 1
2: 2 1
3: 3 1

In the next release, data.table will return both A columns. See here for the discussion.