Returns 0 if the two values are equivalent, 1 if the first value is greater than the second, and -1 if the first value is less than the second.
https://www.mongodb.com/docs/manual/reference/operator/aggregation/cmp/#mongodb-expression-exp.-cmp
Returns 0 if the two values are equivalent, 1 if the first value is greater than the second, and -1 if the first value is less than the second.