10 - Comparison Operators

Comparison Operators

Compare values — equality, inequality, greater/less than.

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Comparison operators are binary logical operators that compare two values and return a boolean result (or a numeric result in the case of $cmp).

$eq and $ne

Test for equality or inequality.

{ $eq: [<expr1>, <expr2>] }   // true if expr1 === expr2
{ $ne: [<expr1>, <expr2>] }   // true if expr1 !== expr2

$gt, $gte, $lt, $lte

Greater than, greater-or-equal, less than, less-or-equal.

{ $gt:  [<expr1>, <expr2>] }  // true if expr1 > expr2
{ $gte: [<expr1>, <expr2>] }  // true if expr1 >= expr2
{ $lt:  [<expr1>, <expr2>] }  // true if expr1 < expr2
{ $lte: [<expr1>, <expr2>] }  // true if expr1 <= expr2

Example: Filter by type and funding threshold

db.projects.aggregate([
  {
    $match: {
      $expr: {
        $and: [
          { $eq: ["$type", "REQUEST_FUNDING_PROJECT"] },
          { $lt: ["$projectFunding", 5000] }
        ]
      }
    }
  },
  { $out: "projectReport" }
]);

Note: When using comparison operators as expressions inside $match, you must wrap them in $expr. See 14 - Variable and Reference Expressions for details on $expr.

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$cmp — Numeric Comparison

Unlike the other comparison operators that return booleans, $cmp returns a number:

Result	Meaning
1	First value is greater than the second
-1	First value is less than the second
0	Values are equal

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

Example

db.tweets.insertMany([
  { _id: 1, item: "abc1", qty: 300 },
  { _id: 2, item: "abc2", qty: 200 }
]);
 
db.tweets.aggregate([
  {
    $project: {
      item: 1,
      _id: 0,
      cmpTo250: { $cmp: ["$qty", 250] }
    }
  }
]);

Output:

{ item: "abc1", cmpTo250:  1 }   // 300 > 250
{ item: "abc2", cmpTo250: -1 }   // 200 < 250

When to use $cmp: When you need the comparison result as a sortable or computable number rather than a boolean — e.g., for scoring or ranking logic.

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Next: 11 - Boolean and Control Flow Operators