Summarize the self-join index while avoiding cartesian product in R data.table

Question

With a 2-column data.table, I\'d like to summarize the pairwise relationships in column 1 by summing the number of shared elements in column 2. In other words,

Answer 1

You already have solution written in SQL so I suggest R package sqldf

Here's code:

library(sqldf)

result <- sqldf("SELECT A.X, B.X, COUNT(A.Y) as N FROM test as A JOIN test as B WHERE A.Y==B.Y GROUP BY A.X, B.X")