Comparison expressions ($eq, $gt in aggregation) in MongoDB

Introduction

Comparison expressions help you check if values are equal or greater than others inside MongoDB aggregation. This lets you filter or create new data based on conditions.

When you want to find documents where a field equals a specific value.

When you need to select documents where a number is greater than a certain limit.

When creating new fields that depend on comparing values inside aggregation.

When filtering data in a pipeline to only keep matching documents.

When grouping data and applying conditions to decide what to include.

Syntax

MongoDB

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

Both $eq and $gt take an array of two expressions to compare.

They return true or false depending on the comparison result.

Examples

Checks if the field status equals "A".

MongoDB

{ $eq: [ "$status", "A" ] }

Checks if the field score is greater than 80.

MongoDB

{ $gt: [ "$score", 80 ] }

Compares two fields age and minAge for equality.

MongoDB

{ $eq: [ "$age", "$minAge" ] }

Sample Program

This aggregation shows each student's name and two new fields: passed is true if their score is greater than 60, and perfectScore is true if their score equals 100.

MongoDB

db.students.aggregate([
  {
    $project: {
      name: 1,
      passed: { $gt: [ "$score", 60 ] },
      perfectScore: { $eq: [ "$score", 100 ] }
    }
  }
])

Important Notes

Remember to use the $ sign before field names inside expressions.

$eq and $gt return boolean values that you can use to filter or create new fields.

These expressions work inside aggregation pipelines, not in simple find queries.

Summary

$eq checks if two values are equal.

$gt checks if the first value is greater than the second.

Use them inside aggregation to compare fields or values and create conditions.