Clojure: Chaining group-by :key with select-keys on remaining keys

I'm trying to understand a simple (as in other languages) workflow with clojure maps.

It basically comes down to this: How can chain these operations?

1. group-by :key on a vector of maps

2. select-keys on remaining maps without the previous key

3. group-by again (0..n times) and select-keys

4. count unique key instances at the end.

See also my previous question: Aggregate and Count in Maps

Example:

Given a vector of maps

(def DATA [{:a "X", :b "M", :c "K", :d 10}
           {:a "Y", :b "M", :c "K", :d 20}
           {:a "Y", :b "M", :c "F", :d 30}
           {:a "Y", :b "P", :c "G", :d 40}])

performing group-by

(defn get-tree-level-1 [] (group-by :a DATA))

yields a map grouped by the value of that particular key.

{ X [{:a X, :b M, :c K, :d 10}],
  Y [{:a Y, :b M, :c K, :d 20}
     {:a Y, :b M, :c F, :d 30}
     {:a Y, :b P, :c G, :d 40}]}  

So far, so good. But what if I want to build a tree-like structure out of the data, which means selecting the remaining keys and ignoring some, select :b and :c and ignore :d, which would yield in the next level:

(def DATA2   [{ :X [{:b "M", :c "K"}],
                :Y [{:b "M", :c "K"}
                    {:b "M", :c "F"}
                    {:b "P", :c "G"}]}])

And finally, counting all instances of the remaining keys (e.g. count all unique values of the :b key under the Y-root):

(def DATA3   [{ :X [{:M  1}],
                :Y [{:M  2}
                    {:P  1}])

I tried doing a select-keys after the group-by, but the result was empty after the first step:

(defn get-proc-sums []
  (into {}
    (map
      (fn [ [k vs] ]
        [k (select-keys vs [:b :c])])
      (group-by :a DATA))))