MongoDB $subtract Operator

The $subtract operator in MongoDB is an aggregation arithmetic operator used to subtract numbers and perform date/time calculations, helping transform fields in MongoDB queries.

• Subtracts two numbers and returns the difference.
• Subtracts two dates and returns the difference in milliseconds.
• Subtracts milliseconds from a date and returns a new date.
• Takes exactly two expressions: { $subtract: [expr1, expr2] }.
• Second value is subtracted from the first.
• Supports number–number, date–date (returns milliseconds), and date–number (subtracts milliseconds from date).

Syntax

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

• $subtract: MongoDB aggregation operator for subtraction.
• [ ... ]: Array containing exactly two values.
• <expression1>: The base value (number or date).
• <expression2>: The value to subtract from the first.
• Type compatibility: Expressions must be compatible (numbers or dates).

Examples of MongoDB $subtract Operator

In the following examples, we are working with:

• Database: GeeksforGeeks
• Collection: Employee
• Document: four documents that contain the details of the employees in the form of field-value pairs.

Example 1: Subtract Numbers

Subtract the value of firstSalary field from the value of secondSalary field using $subtract operator.

Query:

db.Employee.aggregate([
  { $match: { department: "Development" } },
  {
    $project: {
      result: { $subtract: ["$secondSalary", "$firstSalary"] }
    }
  }
])

Output:

Example 2: Subtract Two Dates

Subtract two dates, i.e, the value of projectStartDate field from the value of projectEndDate field using a $subtract operator.

Query:

db.Employee.aggregate([
  { $match: { department: "Testing" } },
  {
    $project: {
      diffResult: { $subtract: ["$projectEndDate", "$projectStartDate"] }
    }
  }
])

Output:

Example 3: Subtract Milliseconds from a Date

Subtract 5*24*60*60000 milliseconds (i.e., 5 days) from the value of projectEndDate field using a $subtract operator.

Query:

db.Employee.aggregate([
  { $match: { department: "Testing" } },
  { $project: {
      newprojectEndDate: { 
        $subtract: ["$projectEndDate", 5 * 24 * 60 * 60000] 
      }
    }
  }
])

Output:

Use Cases of MongoDB $subtract Operator

The $subtract operator is useful for various scenarios, including:

• Age calculation: Find age using the difference between the current date and birth date.
• Time-based calculations: Measure the duration between two events.
• Date manipulation: Add or subtract days, months, or years from a date.
• Financial calculations: Compute differences between amounts (e.g., salary, balance).