How to Use $sum in Aggregation in MongoDB: Syntax and Examples

In MongoDB aggregation, use the $sum operator to add values across documents or fields. It can sum numeric fields or count documents by using $sum: 1. Place $sum inside a $group stage to calculate totals grouped by a key.

Syntax

The $sum operator is used inside the $group stage of an aggregation pipeline. It sums values of a specified field or counts documents.

• { $group: { _id: <grouping_key>, total: { $sum: <expression> } } }
• <grouping_key> is the field or expression to group by.
• <expression> is the field to sum or 1 to count documents.

json

{
  $group: {
    _id: <grouping_key>,
    total: { $sum: <expression> }
  }
}

Example

This example groups sales by product and sums the quantity sold for each product.

javascript

db.sales.aggregate([
  {
    $group: {
      _id: "$product",
      totalQuantity: { $sum: "$quantity" }
    }
  }
])

Output

[
  { "_id": "apple", "totalQuantity": 25 },
  { "_id": "banana", "totalQuantity": 40 },
  { "_id": "orange", "totalQuantity": 30 }
]

Common Pitfalls

Common mistakes include:

• Using $sum outside of $group stage, which causes errors.
• Summing non-numeric fields, resulting in null or errors.
• For counting documents, forgetting to use $sum: 1 inside $group.

javascript

/* Wrong: $sum used outside $group */
db.sales.aggregate([
  { $sum: "$quantity" }
])

/* Right: $sum inside $group */
db.sales.aggregate([
  {
    $group: {
      _id: null,
      totalQuantity: { $sum: "$quantity" }
    }
  }
])

Quick Reference

Usage	Description	Example
Sum field values	Adds numeric values of a field	{ $sum: "$fieldName" }
Count documents	Counts number of documents in group	{ $sum: 1 }
Group by field	Groups documents by a field	{ _id: "$fieldName" }

Key Takeaways

Use $sum inside $group to add values or count documents.

To count documents, use $sum: 1.

Ensure the field summed contains numeric values.

Do not use $sum outside aggregation stages like $group.

Group by a field using _id in $group stage.