When passed 2 rels, returns the rel corresponding to the natural
join. When passed an additional keymap, joins on the corresponding
keys.
;; This simple example shows each element of the first relation joined
;; with each element of the second (because they have no columns in common):
user=> (def first-relation #{ {:a 1} {:a 2} })
user=> (def second-relation #{ {:b 1} {:b 2} })
user=> (join first-relation second-relation)
#{{:b 1, :a 1}
{:b 2, :a 1}
{:b 1, :a 2}
{:b 2, :a 2}}
;; Here's a larger example, in which a relation mainly about animal ownership
;; is joined with a relation about animal personality. The join is used to
;; produce a relation joining information about an animal's personality to
;; that animal.
user=> (def animals #{{:name "betsy" :owner "brian" :kind "cow"}
{:name "jake" :owner "brian" :kind "horse"}
{:name "josie" :owner "dawn" :kind "cow"}})
user=> (def personalities #{{:kind "cow" :personality "stoic"}
{:kind "horse" :personality "skittish"}})
#'user/personalities
user=> (join animals personalities)
#{{:owner "dawn", :name "josie", :kind "cow", :personality "stoic"}
{:owner "brian", :name "betsy", :kind "cow", :personality "stoic"}
{:owner "brian", :name "jake", :kind "horse", :personality "skittish"}}
;; (If cows had two personalities, instead of one, each cow would have
;; two rows in the output.)
;; Suppose `personalities` used `:species` instead of `:kind`:
user=> (def personalities #{{:species "cow" :personality "stoic"}
{:species "horse" :personality "skittish"}})
;; A simple join would produce results like this:
user=> (join animals personalities)
#{{:kind "horse", :owner "brian", :name "jake", :species "cow", :personality "stoic"}
{:kind "cow", :owner "dawn", :name "josie", :species "cow", :personality "stoic"}
{:kind "horse", :owner "brian", :name "jake", :species "horse", :personality "skittish"}
{:kind "cow", :owner "brian", :name "betsy", :species "cow", :personality "stoic"}
{:kind "cow", :owner "dawn", :name "josie", :species "horse", :personality "skittish"}
{:kind "cow", :owner "brian", :name "betsy", :species "horse", :personality "skittish"}}
;; Notice that "Jake" is both a horse and a cow in the first line. That's
;; likely not what you want. You can tell `join` to only produce output
;; where the `:kind` value is the same as the `:species` value like this:
user=> (join animals personalities {:kind :species})
#{{:kind "cow", :owner "dawn", :name "josie", :species "cow", :personality "stoic"}
{:kind "horse", :owner "brian", :name "jake", :species "horse", :personality "skittish"}
{:kind "cow", :owner "brian", :name "betsy", :species "cow", :personality "stoic"}}
;; Notice that the `:kind` and `:species` keys both appear in each output map.
Source
clojure/set.clj:111
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(defn join
"When passed 2 rels, returns the rel corresponding to the natural
join. When passed an additional keymap, joins on the corresponding
keys."
{:added "1.0"}
([xrel yrel] ;natural join
(if (and (seq xrel) (seq yrel))
(let [ks (intersection (set (keys (first xrel))) (set (keys (first yrel))))
[r s] (if (<= (count xrel) (count yrel))
[xrel yrel]
[yrel xrel])
idx (index r ks)]
(reduce (fn [ret x]
(let [found (idx (select-keys x ks))]
(if found
(reduce #(conj %1 (merge %2 x)) ret found)
ret)))
#{} s))
#{}))
([xrel yrel km] ;arbitrary key mapping
(let [[r s k] (if (<= (count xrel) (count yrel))
[xrel yrel (map-invert km)]
[yrel xrel km])
idx (index r (vals k))]
(reduce (fn [ret x]
(let [found (idx (rename-keys (select-keys x (keys k)) k))]
(if found
(reduce #(conj %1 (merge %2 x)) ret found)
ret)))
#{} s))))
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