$min

The $min aggregation operator returns the minimum value of a specified field across a set of documents. This operator is useful for finding the lowest value in a set of documents.

Parameters

• expression: The expression to evaluate. This can be a field path, a variable, or any expression that resolves to a value.

Example (MongoDB Shell)

The following example demonstrates the usage of the $min operator to find the minimum value of the age field across multiple documents.

Create sample documents

db.users.insertMany([
  { name: "John", age: 35 },
  { name: "Jane", age: 28 },
  { name: "Bob", age: 42 },
  { name: "Alice", age: 31 }
]);

Query example

db.users.aggregate([
  { $group: { _id: null, minAge: { $min: "$age" } } },
  { $project: { _id: 0, minAge: 1 } }
])

Output

[ { minAge: 28 } ]

Window operator usage example (MongoDB Shell)

New from version 8.0.2.

The $min operator can also be used as a window operator in the $setWindowFields stage. In this context, it returns the minimum value for the documents in each window. You specify the operator under the output field, and optionally define the window boundaries with a window document.

Note

When used as a window operator in $setWindowFields, $min is limited to 100 MB of intermediate data. An operation that exceeds this limit returns an error.

Create sample documents

db.stockPrices.insertMany([
  { _id: 1, ticker: "ABC", hour: 1, price: 50 },
  { _id: 2, ticker: "ABC", hour: 2, price: 45 },
  { _id: 3, ticker: "ABC", hour: 3, price: 60 },
  { _id: 4, ticker: "ABC", hour: 4, price: 40 },
  { _id: 5, ticker: "XYZ", hour: 1, price: 30 },
  { _id: 6, ticker: "XYZ", hour: 2, price: 35 },
  { _id: 7, ticker: "XYZ", hour: 3, price: 25 }
]);

Query example

The following example partitions the documents by ticker, sorts each partition by hour, and returns the lowest price seen from the start of the partition through the current document.

db.stockPrices.aggregate([
  {
    $setWindowFields: {
      partitionBy: "$ticker",
      sortBy: { hour: 1 },
      output: {
        lowestSoFar: {
          $min: "$price",
          window: { documents: ["unbounded", "current"] }
        }
      }
    }
  }
]);

Output

[
  { "_id": 1, "ticker": "ABC", "hour": 1, "price": 50, "lowestSoFar": 50 },
  { "_id": 2, "ticker": "ABC", "hour": 2, "price": 45, "lowestSoFar": 45 },
  { "_id": 3, "ticker": "ABC", "hour": 3, "price": 60, "lowestSoFar": 45 },
  { "_id": 4, "ticker": "ABC", "hour": 4, "price": 40, "lowestSoFar": 40 },
  { "_id": 5, "ticker": "XYZ", "hour": 1, "price": 30, "lowestSoFar": 30 },
  { "_id": 6, "ticker": "XYZ", "hour": 2, "price": 35, "lowestSoFar": 30 },
  { "_id": 7, "ticker": "XYZ", "hour": 3, "price": 25, "lowestSoFar": 25 }
]

Each document is augmented with lowestSoFar, the running minimum of price within its partition up to and including the current document.

Code examples

To view a code example for using the $min operator, choose the tab for the language that you want to use. The following examples show both accumulator usage (in $group) and window operator usage (in $setWindowFields):

Node.js

const { MongoClient } = require('mongodb');

async function example() {
  const client = await MongoClient.connect('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false');
  const db = client.db('test');

  // Accumulator usage: minimum value across a group
  const users = db.collection('users');
  await users.insertMany([
    { name: "John", age: 35 },
    { name: "Jane", age: 28 },
    { name: "Bob", age: 42 },
    { name: "Alice", age: 31 }
  ]);
  const accumulatorResult = await users.aggregate([
    { $group: { _id: null, minAge: { $min: "$age" } } }
  ]).toArray();
  console.log('Accumulator result:', accumulatorResult);

  // Window operator usage: running minimum within each partition
  const stockPrices = db.collection('stockPrices');
  await stockPrices.insertMany([
    { _id: 1, ticker: "ABC", hour: 1, price: 50 },
    { _id: 2, ticker: "ABC", hour: 2, price: 45 },
    { _id: 3, ticker: "ABC", hour: 3, price: 60 },
    { _id: 4, ticker: "ABC", hour: 4, price: 40 },
    { _id: 5, ticker: "XYZ", hour: 1, price: 30 },
    { _id: 6, ticker: "XYZ", hour: 2, price: 35 },
    { _id: 7, ticker: "XYZ", hour: 3, price: 25 }
  ]);
  const windowResult = await stockPrices.aggregate([
    {
      $setWindowFields: {
        partitionBy: "$ticker",
        sortBy: { hour: 1 },
        output: {
          lowestSoFar: {
            $min: "$price",
            window: { documents: ["unbounded", "current"] }
          }
        }
      }
    }
  ]).toArray();
  console.log('Window result:', windowResult);

  client.close();
}

example();

Python

from pymongo import MongoClient

def example():
    client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
    db = client['test']

    # Accumulator usage: minimum value across a group
    users = db.users
    users.insert_many([
        { "name": "John", "age": 35 },
        { "name": "Jane", "age": 28 },
        { "name": "Bob", "age": 42 },
        { "name": "Alice", "age": 31 }
    ])
    accumulator_result = list(users.aggregate([
        { "$group": { "_id": None, "minAge": { "$min": "$age" } } }
    ]))
    print('Accumulator result:', accumulator_result)

    # Window operator usage: running minimum within each partition
    stock_prices = db.stockPrices
    stock_prices.insert_many([
        { "_id": 1, "ticker": "ABC", "hour": 1, "price": 50 },
        { "_id": 2, "ticker": "ABC", "hour": 2, "price": 45 },
        { "_id": 3, "ticker": "ABC", "hour": 3, "price": 60 },
        { "_id": 4, "ticker": "ABC", "hour": 4, "price": 40 },
        { "_id": 5, "ticker": "XYZ", "hour": 1, "price": 30 },
        { "_id": 6, "ticker": "XYZ", "hour": 2, "price": 35 },
        { "_id": 7, "ticker": "XYZ", "hour": 3, "price": 25 }
    ])
    window_result = list(stock_prices.aggregate([
        {
            "$setWindowFields": {
                "partitionBy": "$ticker",
                "sortBy": { "hour": 1 },
                "output": {
                    "lowestSoFar": {
                        "$min": "$price",
                        "window": { "documents": ["unbounded", "current"] }
                    }
                }
            }
        }
    ]))
    print('Window result:', window_result)

    client.close()

example()