Skip to content

$group

The $group stage aggregates documents by a specified _id expression and applies accumulator operations to grouped documents.

📌 Syntax

{
  "$group": {
    "_id": <expression>,
    "field1": { "<accumulator>": <expression> },
    ...
  }
}

📦 Ecommerce Example – Total Sales and Item Count per Category

{
  "$group": {
    "_id": "$category",
    "totalSales": { "$sum": "$price" },
    "itemCount": { "$sum": 1 }
  }
}

Input Documents

[
  { "product": "Laptop", "category": "Electronics", "price": 900 },
  { "product": "Phone", "category": "Electronics", "price": 600 },
  { "product": "Shoes", "category": "Fashion", "price": 75 },
  { "product": "Book", "category": "Books", "price": 20 }
]

Output Documents

[
  { "_id": "Electronics", "totalSales": 1500, "itemCount": 2 },
  { "_id": "Fashion", "totalSales": 75, "itemCount": 1 },
  { "_id": "Books", "totalSales": 20, "itemCount": 1 }
]

🔁 Group by Composite Keys

{
  "$group": {
    "_id": { "category": "$category", "brand": "$brand" },
    "count": { "$sum": 1 }
  }
}

➕ Supported Accumulators

Accumulator Description
$sum Total or count
$avg Average value
$min / $max Extremes
$push Array of values
$addToSet Unique values
$first / $last First/last by input order

🎯 Ecommerce Accumulator Examples

{
  "$group": {
    "_id": "$category",
    "avgPrice": { "$avg": "$price" },
    "minPrice": { "$min": "$price" },
    "maxPrice": { "$max": "$price" },
    "allBrands": { "$push": "$brand" },
    "uniqueBrands": { "$addToSet": "$brand" },
    "firstItem": { "$first": "$product" },
    "lastItem": { "$last": "$product" }
  }
}

🔧 Operators in Expressions

Operators may appear in _id and value expressions.

Operator Example
$toUpper { "$toUpper": "$category" }
$cond { "$cond": { "if": ..., "then": ..., "else": ... } }
$convert Convert field to number or string
$multiply Pre-process price with quantity

Example – Group by Uppercase Category and Conditional Total

{
  "$group": {
    "_id": { "$toUpper": "$category" },
    "total": {
      "$sum": {
        "$cond": {
          "if": { "$gte": ["$price", 100] },
          "then": "$price",
          "else": 0
        }
      }
    }
  }
}

✅ Best Practices

  • Use $sum: 1 to count documents
  • Use $cond or $map to pre-process values
  • Output field names can differ from input fields