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Aggregation Framework Operators

New in version 2.2.

Pipeline Operators

Note

The aggregation pipeline cannot operate on values of the following types: Symbol, MinKey, MaxKey, DBRef, Code, and CodeWScope.

Pipeline operators appear in an array. Documents pass through the operators in a sequence.

Name Description
$project Reshapes a document stream. $project can rename, add, or remove fields as well as create computed values and sub-documents.
$match Filters the document stream, and only allows matching documents to pass into the next pipeline stage. $match uses standard MongoDB queries.
$limit Restricts the number of documents in an aggregation pipeline.
$skip Skips over a specified number of documents from the pipeline and returns the rest.
$unwind Takes an array of documents and returns them as a stream of documents.
$group Groups documents together for the purpose of calculating aggregate values based on a collection of documents.
$sort Takes all input documents and returns them in a stream of sorted documents.
$geoNear Returns an ordered stream of documents based on proximity to a geospatial point.

Expression Operators

Expression operators calculate values within the Pipeline Operators.

$group Operators

Name Description
$addToSet Returns an array of all the unique values for the selected field among for each document in that group.
$first Returns the first value in a group.
$last Returns the last value in a group.
$max Returns the highest value in a group.
$min Returns the lowest value in a group.
$avg Returns an average of all the values in a group.
$push Returns an array of all values for the selected field among for each document in that group.
$sum Returns the sum of all the values in a group.

Boolean Operators

These operators accept Booleans as arguments and return Booleans as results.

The operators convert non-Booleans to Boolean values according to the BSON standards. Here, null, undefined, and 0 values become false, while non-zero numeric values, and all other types, such as strings, dates, objects become true.

Name Description
$and Returns true only when all values in its input array are true.
$or Returns true when any value in its input array are true.
$not Returns the boolean value that is the opposite of the input value.

Comparison Operators

These operators perform comparisons between two values and return a Boolean, in most cases reflecting the result of the comparison.

All comparison operators take an array with a pair of values. You may compare numbers, strings, and dates. Except for $cmp, all comparison operators return a Boolean value. $cmp returns an integer.

Name Description
$cmp Compares two values and returns the result of the comparison as an integer.
$eq Takes two values and returns true if the values are equivalent.
$gt Takes two values and returns true if the first is larger than the second.
$gte Takes two values and returns true if the first is larger than or equal to the second.
$lt Takes two values and returns true if the second value is larger than the first.
$lte Takes two values and returns true if the second value is larger than or equal to the first.
$ne Takes two values and returns true if the values are not equivalent.

Arithmetic Operators

Arithmetic operators support only numbers.

Name Description
$add Computes the sum of an array of numbers.
$divide Takes two numbers and divides the first number by the second.
$mod Takes two numbers and calculates the modulo of the first number divided by the second.
$multiply Computes the product of an array of numbers.
$subtract Takes two numbers and subtracts the second number from the first.

String Operators

String operators manipulate strings within projection expressions.

Name Description
$concat Concatenates two strings.
$strcasecmp Compares two strings and returns an integer that reflects the comparison.
$substr Takes a string and returns portion of that string.
$toLower Converts a string to lowercase.
$toUpper Converts a string to uppercase.

Date Operators

Date operators take a “Date” typed value as a single argument and return a number.

Name Description
$dayOfYear Converts a date to a number between 1 and 366.
$dayOfMonth Converts a date to a number between 1 and 31.
$dayOfWeek Converts a date to a number between 1 and 7.
$year Converts a date to the full year.
$month Converts a date into a number between 1 and 12.
$week Converts a date into a number between 0 and 53
$hour Converts a date into a number between 0 and 23.
$minute Converts a date into a number between 0 and 59.
$second Converts a date into a number between 0 and 59. May be 60 to account for leap seconds.
$millisecond Returns the millisecond portion of a date as an integer between 0 and 999.

Conditional Expressions

Name Description
$cond A ternary operator that evaluates one expression, and depending on the result returns the value of one following expressions.
$ifNull Evaluates an expression and returns a value.