$join does not work if the 1st element in the second dataset does not have a corresponding element in the first dataset

incanter-join.clj

(ns experiment.incanter
  (:require [incanter.core :as i]))


(def join-dataset-1 (i/dataset [:id :vehicle-type]
                               [[1 "car"]
                                [2 "motor-bike"]
                                [3 "train"]]))
;;;(i/view join-dataset-1)
(def join-dataset-2 (i/dataset [:id :number-wheels]
                               [[1 4]
                                [2 2]
                                [3 "many"]]))
;;;(i/view join-dataset-2)
;;; (->> (i/$join [:id :id] join-dataset-2 join-dataset-1) i/view)
(def join-dataset-3 (i/dataset [:id :number-wheels]
                               [;;[1 4]
                                [2 2]
                                [3 "many"]]))
;;;(i/view join-dataset-3)
;;(->> (i/$join [:id :id] join-dataset-3 join-dataset-1) i/view)
(def join-dataset-4 (i/dataset [:id :number-wheels]
                               [[1 4]
                                ;;[2 2]
                                [3 "many"]]))
;;;(i/view join-dataset-4)
;;(->> (i/$join [:id :id] join-dataset-4 join-dataset-1) i/view)