Question

I have 2 data structures like the ones below

(ns test)

(def l 
  [{:name "Sean" :age 27} 
   {:name "Ross" :age 27}
   {:name "Brian" :age 22}])

(def r 
  [{:owner "Sean" :item "Beer" }
   {:owner "Sean" :item "Pizza"}
   {:owner "Ross" :item "Computer"} 
   {:owner "Matt" :item "Bike"}])

I want to have get persons who dont own any item . (Brian in this case so [ {:name “Brian” :age 22}]
If this was SQL I would do left outer join or not exists but I not sure how to do this in clojure in more performant way.

Answer 1

While Chuck’s solution is certainly the most sensible one, I find it interesting that it is possible to write a solution in terms of relational algebraic operators using clojure.set:

(require '[clojure.set :as set])
(set/difference (set l)
                (set/project (set/join r l {:owner :name})
                             #{:name :age}))
; => #{{:name "Brian", :age 22}}