MongoDB $cmp Operator

The MongoDB $cmp operator is a powerful tool for comparing two values within documents, commonly used in aggregation pipelines for sorting or conditional operations. It plays a crucial role in sorting, conditional operations, and advanced comparisons inside MongoDB queries

In this article, We will learn about the MongoDB $cmp Operator, its syntax, use cases, and detailed examples to help us leverage its full potential in our MongoDB applications.

What is the MongoDB $cmp Operator?

The MongoDB $cmp operator is used to compare two values and return. It is commonly used in aggregation pipelines for sorting and evaluating conditions within documents. This operator is often utilized in $project, $sort, and conditional expressions within aggregation queries. The comparison follows MongoDB’s BSON type ordering when dealing with values of different types.

• 0 : If both values are equal
• 1 : If the first value is greater than the second
• -1 : If the first value is less than the second

Syntax:

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

Key Terms

• <expression1> – The first value or field to compare.
• <expression2> – The second value or field to compare.

Key Features of the MongoDB $cmp Operator

• Compares Two Values: Returns 0, 1, or -1 based on the comparison result.
• Supports Nested Fields: Can compare fields within embedded documents.
• Handles Different Data Types: Uses MongoDB BSON type ordering when comparing different types.
• Works in Aggregation Pipelines: Used with $project, $sort, and other stages in aggregation.

MongoDB $cmp Operator Examples

Let's explore some real-world examples of the $cmp operator in MongoDB. In the following examples, we are working with:

• Database: GeeksforGeeks
• Collection: example
• Document: two documents that contain the details of the shapes in the form of field-value pairs.

Example 1: Using $cmp to Compare a Field with a Statis Value

In this example, we are comparing the value of the side field with 4 and $cmp operator return 0 which means both values are equal.

Query:

db.example.aggregate([{$match: {name: "Square"}},
... {$project: {result: {$cmp:["$side", 4]}}}])

Output:

Explanation: Since the side field is 4, $cmp returns 0 as both values are equal

Example 2: Using $cmp Operator in the Embedded Document:

In this example, we are comparing the value of the measurement.height field with the value of the measurement.width field and $cmp operator return -1 which means both values of measurement.height field is less than the value of the measurement.width field.

Query:

db.example.aggregate([{$match: {name: "Rectangle"}},
... {$project: {result:
... {$cmp:["$measurement.height", "$measurement.width"]}}}])

Output:

Explanation: Since measurement.height is less than measurement.width, $cmp returns -1

Important Points About MongoDB $cmp Operator

• The $cmp operator is used to compare two values or types and returns the result. It returns -1 if the first value is less than the second, 1 if the first value is greater than the second, and 0 if both values are equal.
• The $cmp operator compares both value and type, using the specified BSON comparison order for values of different types.
• The $cmp operator is used in the aggregation pipeline stages.

Conclusion

The MongoDB $cmp operator offers a straightforward way to compare values within documents, facilitating sorting and conditional operations in aggregation pipelines. Its ability to handle different types and return a numeric result makes it a versatile tool for various comparison needs in MongoDB queries. By mastering $cmp, we can optimize MongoDB queries for data analysis, sorting, and dynamic computations in our applications.