MongoDB — Group

Basic Group

db.users.aggregate([
  { $group: { _id: "$country", count: { $sum: 1 } } }
])

Multiple Fields

db.users.aggregate([
  { $group: { 
    _id: { country: "$country", status: "$status" }, 
    count: { $sum: 1 } 
  }}
])

Accumulators

db.users.aggregate([
  { $group: { 
    _id: "$country", 
    totalAge: { $sum: "$age" },
    avgAge: { $avg: "$age" },
    minAge: { $min: "$age" },
    maxAge: { $max: "$age" },
    count: { $sum: 1 }
  }}
])

Group by Null (All)

db.users.aggregate([
  { $group: { _id: null, avgAge: { $avg: "$age" } } }
])

Push to Array

db.users.aggregate([
  { $group: { 
    _id: "$country", 
    names: { $push: "$name" } 
  }}
])

Mini Practice

1. Group by field
2. Use multiple accumulators
3. Group by multiple fields
4. Push values to array

Up Next

Continue with Lookup — joining collections.