Merge two data frames with non-unique keys

Question

I want to select the rows of data1 if the value of the first column exists in data2:

data1<-data.table(a=c(1,2,3,1),b=c(1,4,6,3))
data2<-data.table(a=c(1,3,1,5,6))


Desired output:
a b 
1 1 
3 6 
1 3 

The merge does not work since the keys are not unique, any other idea?

Answer 1

Here's an approach without joins:

> unique(data1[a %in% data2[,a],])
   a b
1: 1 1
2: 3 6
3: 1 3

Answer 2

We can create a sequence column ('ind') in each of the datasets for the 'a' column and 'join' after setting the key column as 'a' and 'ind'

data1[, ind:=1:.N, a]
data2[, ind:= 1:.N, a]
setkey(data1, a, ind)[data2, nomatch=0][, ind:=NULL]
#   a b
#1: 1 1
#2: 3 6
#3: 1 3