Way to calculate the top x% of users from a database

Question

I was wondering if there was an actually good way to aggregate/calculate percentiles based on all data in a database. For example, data like:

{
'name':'John'
'rank':5
},
{
'name':'John'
'rank':4
},
{
'name':'John'
'rank':5
},
{
'name':'James'
'rank':3
},
{
'name':'Froggy'
'rank':5
},

How would I go about finding a way to calculate the percentile that users achieve a specific rarity compared to others. Example: James falls in the top 10% of users who own rank 5.

Answer 1

emphasized textYou can do something like:

db.collection.aggregate([
  {
    $group: {
      _id: "$name",
      rankFiveCount: {$sum: {$cond: [{$eq: ["$rank", 5]}, 1, 0]}}}
  },
  {
    $setWindowFields: {
      sortBy: {rankFiveCount: -1},
      output: {
        rank: {$rank: {}},
        totalCount: {$count: {}}
      }
    }
  },
  {$match: {_id: wantedUser}},
  {
    $project: {
      name: "$_id",
      _id: 0,
      percentile: {
        $round: {$multiply: [{$divide: ["$rank", "$totalCount"]}, 100]}
      }
    }
  }
])

See how it works on the playground example