MongoDB $lt Operator

What You’ll Learn

The $lt operator checks whether one value is less than another. Use it in $match to filter documents below a threshold, or in expression stages like $project and $cond to compare fields and get a true/false result.

Definition and Usage

In MongoDB, the $lt operator tests whether one value is strictly less than another. In a $match stage, { age: { $lt: 21 } } filters documents where age is less than 21. In an aggregation expression, { $lt: [ "$price", 100 ] } returns true or false for each document. It is one of the core comparison operators alongside $lte, $gt, and $gte.

📝 Syntax

$lt has two forms depending on where you use it:

Query form (in $match or find filters)

{ field: { $lt: <value> } }

Aggregation expression form (in $project, $cond, etc.)

{ $lt: [ <expression1>, <expression2> ] }

Syntax Rules

• Query form — filters documents where the field is less than the given value.
• Expression form — compares two expressions and returns true or false.
• Strictly less than — equal values return false (use $lte to include equals).
• Works with numbers, dates, and strings (BSON comparison order).
• Use expression form inside $project, $addFields, $cond, and $filter.

Examples Gallery

Walk through sample student data, filter by age with $match, and compare fields with the expression form.

[
  { "_id": ObjectId("609c26812e9274a86871bc6a"), "name": "Alice", "age": 20, "grade": "A" },
  { "_id": ObjectId("609c26812e9274a86871bc6b"), "name": "Bob", "age": 25, "grade": "B" },
  { "_id": ObjectId("609c26812e9274a86871bc6c"), "name": "Charlie", "age": 18, "grade": "C" }
]

db.students.aggregate([
  {
    $match: {
      age: { $lt: 21 }
    }
  }
])

{ "_id": ObjectId("609c26812e9274a86871bc6a"), "name": "Alice", "age": 20, "grade": "A" }
{ "_id": ObjectId("609c26812e9274a86871bc6c"), "name": "Charlie", "age": 18, "grade": "C" }

db.products.find({
  price: { $lt: 50 }
})

// Returns all products where price < 50
// price: 30  → included
// price: 50  → excluded (not strictly less)
// price: 99  → excluded

db.products.aggregate([
  {
    $project: {
      name: 1,
      price: 1,
      isBudget: {
        $lt: [ "$price", 100 ]
      }
    }
  }
])

// price: 75  → isBudget: true
// price: 100 → isBudget: false
// price: 150 → isBudget: false

db.students.aggregate([
  {
    $project: {
      name: 1,
      age: 1,
      category: {
        $cond: [
          { $lt: [ "$age", 18 ] },
          "minor",
          "adult"
        ]
      }
    }
  }
])

db.students.aggregate([
  {
    $match: {
      age: { $gt: 18, $lt: 25 }
    }
  }
])

// Matches students older than 18 AND younger than 25
// Alice (20) → included
// Bob (25)   → excluded (25 is not < 25)
// Charlie (18) → excluded (18 is not > 18)

🚀 Use Cases

• Age and eligibility filtering — find users below a minimum age or senior discount threshold.
• Price caps — retrieve products or orders under a budget limit.
• Date filtering — select records created before a specific date using { createdAt: { $lt: date } }.
• Conditional logic — use as the if condition inside $cond for tiered labels and alerts.

💡 Best Practices