R data.table join: SQL “select *” alike syntax in joined tables?

Question

I have two data.tables with many fields.

I want to join the two tables, add some calculated fields and append all other fields from the first, second or both tables

Answer 1

This should precisely answer your need.
It uses very powerful R feature called computing on the language (or meta programming) well described in official R Language Definition manual. This is an exceptional feature of R language and should not be forgotten IMO.

library(data.table)
DT1 = data.table(x=c("c", "a", "b", "a", "b"), a=1:5)
DT2 = data.table(x=c("d", "c", "b"), b=6:8)

jj = as.call(c(
    list(as.name(".")),
    list(sum = quote(a+b)),
    lapply(unique(c(names(DT1), names(DT2))), as.name)
))
print(jj)
#.(sum = a + b, x, a, b)
DT1[DT2, eval(jj), on="x"]
#   sum x  a b
#1:  NA d NA 6
#2:   8 c  1 7
#3:  11 b  3 8
#4:  13 b  5 8