MongoDB $cmp Operator

MongoDB provides different types of comparison expression operators that are used in the aggregation pipeline stages $cmp operator is one of them. This operator is used to perform a comparison between two values and return the following result according to the condition:

• If the first value is greater than the second value, then this operator will return 1. 
• If the first value is less than the second value, then this operator will return -1. 
• If both the values are equal, then this operator will return 0.

Syntax: 

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

Examples:

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.

Using $cmp Operator:

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.

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

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.

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

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