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

In MongoDB, $min and $max are used in update operations to conditionally set a field to a lower or higher value, respectively. They can also be used in aggregation pipelines to find minimum or maximum values across documents.

Syntax

The $min and $max operators are primarily used in update commands to update a field only if the new value is less than ($min) or greater than ($max) the current field value.

Basic syntax in an update operation:

{
  $min: { :  }
}

{
  $max: { :  }
}

Where:

• <field> is the document field to update.
• <value> is the value to compare and possibly set.

In aggregation, $min and $max are used as accumulator operators to find minimum or maximum values across documents.

mongodb

db.collection.updateOne(
  { _id: 1 },
  { $min: { score: 50 } }
)

db.collection.updateOne(
  { _id: 1 },
  { $max: { score: 90 } }
)

Example

This example shows how to use $min and $max in update operations to conditionally update a user's score.

mongodb

use testdb;

// Insert a sample document
db.users.insertOne({ _id: 1, name: "Alice", score: 75 });

// Try to update score to 50 using $min (will update because 50 < 75)
db.users.updateOne(
  { _id: 1 },
  { $min: { score: 50 } }
);

// Check updated document
printjson(db.users.findOne({ _id: 1 }));

// Try to update score to 90 using $max (will update because 90 > 50)
db.users.updateOne(
  { _id: 1 },
  { $max: { score: 90 } }
);

// Check updated document
printjson(db.users.findOne({ _id: 1 }));

Output

{ "_id" : 1, "name" : "Alice", "score" : 50 }
{ "_id" : 1, "name" : "Alice", "score" : 90 }

Common Pitfalls

Common mistakes when using $min and $max include:

• Using them outside update operations where they are not supported.
• Expecting them to work like simple assignment operators; they only update if the condition is met.
• Not considering the field type; comparisons depend on the BSON type ordering.

Example of a wrong usage and the correct way:

mongodb

// Wrong: Using $min in a find query (invalid)
db.collection.find({ $min: { score: 50 } });

// Right: Use $min in update operation
db.collection.updateOne(
  { _id: 1 },
  { $min: { score: 50 } }
);

Quick Reference

Operator	Usage	Effect
$min	Update operator	Sets field to value if value is less than current field value
$max	Update operator	Sets field to value if value is greater than current field value
$min	Aggregation accumulator	Returns minimum value from group of documents
$max	Aggregation accumulator	Returns maximum value from group of documents

Key Takeaways

Use $min and $max in update operations to conditionally update fields based on comparison.

$min updates the field only if the new value is less than the current value.

$max updates the field only if the new value is greater than the current value.

In aggregation, $min and $max find minimum or maximum values across documents.

Avoid using $min and $max outside their supported contexts like update or aggregation.