How to Use $switch in Aggregation in MongoDB

Use the $switch operator inside a MongoDB aggregation pipeline to evaluate multiple conditions and return a value for the first true case. It works like a switch-case statement, with branches for conditions and a default value if no conditions match.

Syntax

The $switch operator has this structure:

• branches: An array of objects, each with case (a condition) and then (the value to return if the case is true).
• default: The value to return if none of the cases are true.

json

{
  $switch: {
    branches: [
      { case: <expression>, then: <value> },
      { case: <expression>, then: <value> },
      ...
    ],
    default: <value>
  }
}

Example

This example shows how to use $switch to categorize scores into grades inside an aggregation pipeline.

mongodb

db.students.aggregate([
  {
    $project: {
      name: 1,
      score: 1,
      grade: {
        $switch: {
          branches: [
            { case: { $gte: ["$score", 90] }, then: "A" },
            { case: { $gte: ["$score", 80] }, then: "B" },
            { case: { $gte: ["$score", 70] }, then: "C" },
            { case: { $gte: ["$score", 60] }, then: "D" }
          ],
          default: "F"
        }
      }
    }
  }
])

Output

[
  { "_id": ObjectId("..."), "name": "Alice", "score": 85, "grade": "B" },
  { "_id": ObjectId("..."), "name": "Bob", "score": 72, "grade": "C" },
  { "_id": ObjectId("..."), "name": "Charlie", "score": 59, "grade": "F" }
]

Common Pitfalls

• Forgetting to include the default value can cause null results when no cases match.
• Conditions must be valid MongoDB expressions; using incorrect syntax causes errors.
• The branches array is evaluated in order; the first true case stops evaluation.

json

/* Wrong: missing default, so unmatched cases return null */
{
  $switch: {
    branches: [
      { case: { $eq: ["$status", "active"] }, then: "Active" }
    ],
    default: null
  }
}

/* Right: add default to handle unmatched cases */
{
  $switch: {
    branches: [
      { case: { $eq: ["$status", "active"] }, then: "Active" }
    ],
    default: "Inactive"
  }
}

Quick Reference

Part	Description
$switch	Operator to evaluate multiple conditions and return a value.
branches	Array of { case: condition, then: value } objects.
case	Condition expression to test.
then	Value returned if the case is true.
default	Value returned if no cases match.

Key Takeaways

Use $switch to handle multiple conditional branches in aggregation pipelines.

Always include a default value to avoid null results when no cases match.

Branches are checked in order; the first true case determines the output.

Conditions inside case must be valid MongoDB expressions.

$switch helps simplify complex conditional logic inside aggregation stages.