09 - Arithmetic Operators

Arithmetic Operators

Do math — add, subtract, multiply, divide, and get absolute values.

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Arithmetic operators take numeric values as input and return a single numeric result.

$add and $subtract

Compute the sum or difference of values. $add accepts two or more arguments.

// Syntax
{ $add:      [<expr1>, <expr2>, ...] }
{ $subtract: [<expr1>, <expr2>] }

Example: Sum multiple research fields

db.subprojects.aggregate([
  {
    $project: {
      researchFocus: {
        $add: [
          "$theoreticalResearch",
          "$focusResearch",
          "$appliedResearch"
        ]
      }
    }
  },
  { $out: "subprojectReport" }
]);

Example: Subtract a fixed value

{
  value: { $subtract: ["$projectFunding", 10] }
}

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$multiply

Computes the product of two or more values.

{ $multiply: [<expr1>, <expr2>, ...] }

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$divide

Divides the first value by the second.

{ $divide: [<expr1>, <expr2>] }

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$abs

Returns the absolute value of a number.

{ $abs: <expression> }

Example

db.projects.aggregate([
  {
    $project: {
      projectFunding: { $abs: "$projectFunding" },
      title: 1
    }
  },
  { $out: "projectReport" }
]);

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Nesting Arithmetic Expressions

The real power comes from combining operators:

// (3 + 3) × 10 = 60
{ $multiply: [{ $add: [3, 3] }, 10] }
 
// (4 + 4) / (2 × 3) = 1.33...
{
  $divide: [
    { $add: [4, 4] },
    { $multiply: [2, 3] }
  ]
}

Full pipeline example

db.projects.aggregate([
  {
    $addFields: {
      projectFunding: {
        $multiply: [
          { $add: ["$projectFunding", 10] },
          3
        ]
      }
    }
  },
  { $out: "projectReport" }
]);

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Next: 10 - Comparison Operators