Joining two data.tables in R based on multiple keys and duplicate entries

Question

I am trying to join two data.tables in R base don multiple setkeys and which have repeated entries. As an example

>DT1
ID  state Month Day Year
1   IL             


        

Answer 1

Just helped a friend with this (he couldn't find a good Stack Overflow answer) so I figured this question needed a more complete "toy" answer.

Here's a couple of simple data tables with one mismatched key:

dt1 <- data.table(a = LETTERS[1:5],b=letters[1:5],c=1:5)
dt2 <- data.table(c = LETTERS[c(1:4,6)],b=letters[1:5],a=6:10)

And here's several multiple key merge options:

merge(dt1,dt2,by.x=c("a","b"),by.y=c("c","b")) #Inner Join
merge(dt1,dt2,by.x=c("a","b"),by.y=c("c","b"),all=T) #Outer Join

setkey(dt1,a,b)
setkey(dt2,c,b)

dt2[dt1] #Left Join (if dt1 is the "left" table)
dt1[dt2] #Right Join (if dt1 is the "left" table)