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$group & Accumulators

$group & Accumulators

$group groups documents by a key and calculates aggregate values.

Basic Syntax

{
  $group: {
    _id: "$fieldToGroupBy",
    outputField: { $accumulator: "$valueField" }
  }
}

Common Accumulators

Accumulator	Meaning
`$sum`	Adds values or counts documents
`$avg`	Calculates average
`$min`	Finds minimum
`$max`	Finds maximum
`$push`	Adds values to an array
`$addToSet`	Adds unique values to an array
`$first`	First value in group
`$last`	Last value in group

Count Documents Per Status

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

Revenue Per Customer

db.orders.aggregate([
  { $match: { status: "completed" } },
  {
    $group: {
      _id: "$userId",
      totalSpent: { $sum: "$totalAmount" },
      averageOrderValue: { $avg: "$totalAmount" },
      orderCount: { $sum: 1 },
      highestOrder: { $max: "$totalAmount" }
    }
  },
  { $sort: { totalSpent: -1 } }
])

Group By Multiple Fields

db.orders.aggregate([
  {
    $group: {
      _id: {
        year: { $year: "$createdAt" },
        month: { $month: "$createdAt" },
        status: "$status"
      },
      count: { $sum: 1 },
      revenue: { $sum: "$totalAmount" }
    }
  }
])

Overall Total

Use _id: null when you want one result for the whole collection.

db.orders.aggregate([
  {
    $group: {
      _id: null,
      totalRevenue: { $sum: "$totalAmount" },
      totalOrders: { $sum: 1 }
    }
  }
])

`$push` vs `$addToSet`

$push keeps duplicates:

{ $group: { _id: "$category", brands: { $push: "$brand" } } }

$addToSet keeps unique values:

{ $group: { _id: "$category", brands: { $addToSet: "$brand" } } }