MongoDB $eq Operator

What You’ll Learn

The $eq operator checks whether two values are equal. Use it in $match to filter documents by exact values, or in expression stages like $project and $cond to compare fields and get a true/false result.

Definition and Usage

In MongoDB, the $eq operator tests equality in two contexts. In a $match stage, { salary: { $eq: 60000 } } filters documents where salary equals 60000. In an aggregation expression, { $eq: [ "$department", "Engineering" ] } returns true or false for each document. It is one of the most fundamental comparison tools in MongoDB.

📝 Syntax

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

Query form (in $match or find filters)

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

// Shorthand (equivalent):
{ field: <value> }

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

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

Syntax Rules

• Query form — filters documents where the field equals the given value.
• Expression form — compares two expressions and returns true or false.
• Both operands must be the same BSON type for a match (no automatic type conversion).
• Use expression form inside $project, $addFields, $cond, and $filter.
• Pair with $and and $or to build compound conditions.

Examples Gallery

Walk through sample employee data, filter by exact salary with $match, and compare fields with the expression form.

[
  { "_id": ObjectId("609c26812e9274a86871bc6a"), "name": "Alice", "department": "Engineering", "salary": 60000 },
  { "_id": ObjectId("609c26812e9274a86871bc6b"), "name": "Bob", "department": "Sales", "salary": 50000 },
  { "_id": ObjectId("609c26812e9274a86871bc6c"), "name": "Charlie", "department": "Engineering", "salary": 65000 }
]

db.employees.aggregate([
  {
    $match: {
      salary: { $eq: 60000 }
    }
  }
])

{ "_id": ObjectId("609c26812e9274a86871bc6a"), "name": "Alice", "department": "Engineering", "salary": 60000 }

db.employees.aggregate([
  {
    $project: {
      name: 1,
      department: 1,
      isEngineering: {
        $eq: [ "$department", "Engineering" ]
      }
    }
  }
])

// Alice   → isEngineering: true
// Bob     → isEngineering: false
// Charlie → isEngineering: true

db.inventory.aggregate([
  {
    $project: {
      item: 1,
      expected: 1,
      actual: 1,
      isMatch: {
        $eq: [ "$expected", "$actual" ]
      }
    }
  }
])

// expected: 100, actual: 100 → isMatch: true
// expected: 50,  actual: 48  → isMatch: false

db.employees.aggregate([
  {
    $project: {
      name: 1,
      department: 1,
      teamLabel: {
        $cond: [
          { $eq: [ "$department", "Engineering" ] },
          "Tech Team",
          "Business Team"
        ]
      }
    }
  }
])

🚀 Use Cases

• Data filtering — select documents that match an exact field value in $match.
• Conditional logic — use as the if condition inside $cond for categorization and labeling.
• Data validation — compare expected vs actual fields to flag mismatches.
• Boolean flags — add true/false fields in $project for downstream filtering or reporting.

💡 Best Practices