Clojure: Aggregate and Count in Maps

I guess this question qualifies as an entry-level clojure problem. I basically have troubles processing a clojure map multiple times and extract different kinds of data.

Given a map like this, I'm trying to count entries based on multiple nested keys:

[
  {
    "a": "X",
    "b": "M",
    "c": 188
  },
  {
    "a": "Y",
    "b": "M",
    "c": 165
  },
  {
    "a": "Y",
    "b": "M",
    "c": 313
  },
  {
    "a": "Y",
    "b": "P",
    "c": 188
  }
]

First, I want to group the entries by the a-key values:

{
  "X" : [
    {
      "b": "M",
      "c": 188
    }
  ],
  "Y" : [
    {
      "b": "M",
      "c": 165
    },
    {
      "b": "M",
      "c": 313
    },
    {
      "b": "P",
      "c": 188
    }
  ]
}

Second, I want to assume values of b-keys as duplicates and ignore the remaining keys:

{
  "X" : [
    {
      "b": "M"
    }
  ],
  "Y" : [
    {
      "b": "M"
    }
    {
      "b": "P"
    }
  ]
}

Then, simply count all instances of the b-key:

{
  "X" : 1,
  "Y" : 2
}

As I'm getting the data through monger, I defined:

(defn db-query
  ([coll-name]
     (with-open [conn (mg/connect)]
       (doall (mc/find-maps (mg/get-db conn db-name) coll-name))))

and then hitting the roadblock:

(defn get-sums [request]
  (->> (db-query "data")
       (group-by :a)
       (into {})
        keys))

How could I continue from here?