library(data.table)
library(magrittr)

## generage fake dt1
dt1 <-
  data.table(
    a = rep(1:50000, each = 3),
    b = rnorm(150000)
  )
dt2 <-
  data.table(
    a = c(2, 5, 6, 25468)
  )

cat("\n\nPrint dt1\n")
print(dt1)
cat("\n\nPrint dt2\n")
print(dt2)
cat("\n\nPrint merging dt1 with dt2 by `a`\n")
merge(dt1, dt2, by = "a") # 此列為所求