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

In MongoDB aggregation, use the $max operator to find the highest value of a specified field within grouped documents. It is typically used inside the $group stage to return the maximum value for each group or across the entire collection.

Syntax

The $max operator is used inside the $group stage of an aggregation pipeline. It takes a field expression and returns the maximum value found in that group.

• _id: Defines the group key.
• maxValue: The field name to store the maximum value.
• $max: The operator to find the maximum value of the specified field.

json

{
  $group: {
    _id: "<grouping_field>",
    maxValue: { $max: "$<field_to_compare>" }
  }
}

Example

This example groups documents by the category field and finds the maximum price in each category.

mongodb

db.products.aggregate([
  {
    $group: {
      _id: "$category",
      maxPrice: { $max: "$price" }
    }
  }
])

Output

[
  { "_id": "electronics", "maxPrice": 1200 },
  { "_id": "books", "maxPrice": 45 },
  { "_id": "clothing", "maxPrice": 150 }
]

Common Pitfalls

Common mistakes when using $max include:

• Using $max outside of a $group stage, which will cause an error.
• Not specifying the correct field path with $ prefix, leading to unexpected results.
• Grouping by null or an incorrect field, which aggregates all documents into one group unintentionally.

mongodb

/* Wrong usage: $max outside $group */
db.products.aggregate([
  { $max: "$price" }
])

/* Correct usage: $max inside $group */
db.products.aggregate([
  {
    $group: {
      _id: null,
      maxPrice: { $max: "$price" }
    }
  }
])

Quick Reference

Operator	Purpose	Usage Context
$max	Finds the maximum value of a field	Inside $group stage
$min	Finds the minimum value of a field	Inside $group stage
$avg	Calculates average value	Inside $group stage
$sum	Calculates sum of values	Inside $group stage

Key Takeaways

Use $max inside the $group stage to find the highest value in grouped documents.

Always prefix the field name with $ when using $max to specify the field path.

Grouping by the correct field is essential to get meaningful maximum values.

Using $max outside $group will cause errors in the aggregation pipeline.