piece set popularity

gistfile1.txt

ns351169(mongod-3.0.3)[PRIMARY:lichess-rs0] lichess> db.pref.aggregate({$group:{_id:'$pieceSet',nb:{$sum:1}}},{$sort:{nb:-1}})                                        [3/1990]
{
  "result": [
    {
      "_id": "cburnett",
      "nb": 110817
    },
    {
      "_id": null,
      "nb": 49492
    },
    {
      "_id": "merida",
      "nb": 10877
    },
    {
      "_id": "alpha",
      "nb": 6889
    },
    {
      "_id": "pirouetti",
      "nb": 896
    },
    {
      "_id": "fantasy",
      "nb": 653
    },
    {
      "_id": "spatial",
      "nb": 612
    },
    {
      "_id": "chess7",
      "nb": 530
    },
    {
      "_id": "reillycraig",
      "nb": 166
    },
    {
      "_id": "shapes",
      "nb": 86
    }
  ]