Transformation

(defn normalize
  "Transforms a tabular form to a normalised hierarchical form."
  ([kw values-kw coll]
    (normalize kw values-kw [] coll))
  ([kw values-kw promoted-keys coll]
    (normalize kw values-kw promoted-keys identity coll))
  ([kw values-kw promoted-keys f coll]
    (map 
      (fn [[k v]] 
        (merge {kw k 
                values-kw (f (map #(apply dissoc % (conj promoted-keys kw)) v))}
          (select-keys (first v) promoted-keys)))
        (group-by kw coll))))

(def foo [{:a 1 :a2 9 :b 2 :b2 7 :c 3} 
          {:a 1 :a2 9 :b 2 :b2 7 :c 6} 
          {:a 1 :a2 9 :b 5 :b2 6 :c 7} 
          {:a 2 :a2 9 :b 8 :b2 5 :c 9}])

(normalize :a :bs [:a2] (partial normalize :b :cs [:b2]) foo)
;; ({:a2 9, 
;;   :a 1, 
;;   :bs ({:b2 7, 
;;         :b 2, 
;;         :cs ({:c 3} 
;;              {:c 6})} 
;;        {:b2 6, 
;;         :b 5, 
;;         :cs ({:c 7})})} 
;;  {:a2 9, 
;;   :a 2, 
;;   :bs ({:b2 5, 
;;         :b 8, 
;;         :cs ({:c 9})})})