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Aggregation Pipeline Operators

Note

For details on specific operator, including syntax and examples, click on the specific operator to go to its reference page.

Expression Operators

These expression operators are available to construct expressions for use in the aggregation pipeline stages.

Operator expressions are similar to functions that take arguments. In general, these expressions take an array of arguments and have the following form:

{ <operator>: [ <argument1>, <argument2> ... ] }

If operator accepts a single argument, you can omit the outer array designating the argument list:

{ <operator>: <argument> }

To avoid parsing ambiguity if the argument is a literal array, you must wrap the literal array in a $literal expression or keep the outer array that designates the argument list.

Name Description
$abs Returns the absolute value of a number.
$add Adds numbers to return the sum, or adds numbers and a date to return a new date. If adding numbers and a date, treats the numbers as milliseconds. Accepts any number of argument expressions, but at most, one expression can resolve to a date.
$addToSet

Returns an array of unique expression values for each group. Order of the array elements is undefined.

Available in $group stage only.

$allElementsTrue Returns true if no element of a set evaluates to false, otherwise, returns false. Accepts a single argument expression.
$and Returns true only when all its expressions evaluate to true. Accepts any number of argument expressions.
$anyElementTrue Returns true if any elements of a set evaluate to true; otherwise, returns false. Accepts a single argument expression.
$arrayElemAt Returns the element at the specified array index.
$arrayToObject Converts an array of key value pairs to a document.
$avg

Returns an average of numerical values. Ignores non-numeric values.

Changed in version 3.2: Available in both $group and $project stages.

$ceil Returns the smallest integer greater than or equal to the specified number.
$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.
$concat Concatenates any number of strings.
$concatArrays Concatenates arrays to return the concatenated array.
$cond A ternary operator that evaluates one expression, and depending on the result, returns the value of one of the other two expressions. Accepts either three expressions in an ordered list or three named parameters.
$dateFromParts Constructs a BSON Date object given the date’s constituent parts.
$dateToParts Returns a document containing the constituent parts of a date.
$dateFromString Returns a date/time as a date object.
$dateToString Returns the date as a formatted string.
$dayOfMonth Returns the day of the month for a date as a number between 1 and 31.
$dayOfWeek Returns the day of the week for a date as a number between 1 (Sunday) and 7 (Saturday).
$dayOfYear Returns the day of the year for a date as a number between 1 and 366 (leap year).
$divide Returns the result of dividing the first number by the second. Accepts two argument expressions.
$eq Returns true if the values are equivalent.
$exp Raises e to the specified exponent.
$filter Selects a subset of the array to return an array with only the elements that match the filter condition.
$first

Returns a value from the first document for each group. Order is only defined if the documents are in a defined order.

Available in $group stage only.

$floor Returns the largest integer less than or equal to the specified number.
$gt Returns true if the first value is greater than the second.
$gte Returns true if the first value is greater than or equal to the second.
$hour Returns the hour for a date as a number between 0 and 23.
$ifNull Returns either the non-null result of the first expression or the result of the second expression if the first expression results in a null result. Null result encompasses instances of undefined values or missing fields. Accepts two expressions as arguments. The result of the second expression can be null.
$in Returns a boolean indicating whether a specified value is in an array.
$indexOfArray Searches an array for an occurence of a specified value and returns the array index of the first occurence. If the substring is not found, returns -1.
$indexOfBytes Searches a string for an occurence of a substring and returns the UTF-8 byte index of the first occurence. If the substring is not found, returns -1.
$indexOfCP Searches a string for an occurence of a substring and returns the UTF-8 code point index of the first occurence. If the substring is not found, returns -1.
$isArray Determines if the operand is an array. Returns a boolean.
$isoDayOfWeek Returns the weekday number in ISO 8601 format, ranging from 1 (for Monday) to 7 (for Sunday).
$isoWeek Returns the week number in ISO 8601 format, ranging from 1 to 53. Week numbers start at 1 with the week (Monday through Sunday) that contains the year’s first Thursday.
$isoWeekYear Returns the year number in ISO 8601 format. The year starts with the Monday of week 1 (ISO 8601) and ends with the Sunday of the last week (ISO 8601).
$last

Returns a value from the last document for each group. Order is only defined if the documents are in a defined order.

Available in $group stage only.

$let

Defines variables for use within the scope of a subexpression and returns the result of the subexpression. Accepts named parameters.

Accepts any number of argument expressions.

$literal Return a value without parsing. Use for values that the aggregation pipeline may interpret as an expression. For example, use a $literal expression to a string that starts with a $ to avoid parsing as a field path.
$ln Calculates the natural log of a number.
$log Calculates the log of a number in the specified base.
$log10 Calculates the log base 10 of a number.
$lt Returns true if the first value is less than the second.
$lte Returns true if the first value is less than or equal to the second.
$map Applies a subexpression to each element of an array and returns the array of resulting values in order. Accepts named parameters.
$max

Returns the highest expression value for each group.

Changed in version 3.2: Available in both $group and $project stages.

$mergeObjects Combines multiple documents into a single document.
$meta Access text search metadata.
$min

Returns the lowest expression value for each group.

Changed in version 3.2: Available in both $group and $project stages.

$millisecond Returns the milliseconds of a date as a number between 0 and 999.
$minute Returns the minute for a date as a number between 0 and 59.
$mod Returns the remainder of the first number divided by the second. Accepts two argument expressions.
$month Returns the month for a date as a number between 1 (January) and 12 (December).
$multiply Multiplies numbers to return the product. Accepts any number of argument expressions.
$ne Returns true if the values are not equivalent.
$not Returns the boolean value that is the opposite of its argument expression. Accepts a single argument expression.
$objectToArray Converts a document to an array of documents representing key-value pairs.
$or Returns true when any of its expressions evaluates to true. Accepts any number of argument expressions.
$pow Raises a number to the specified exponent.
$push

Returns an array of expression values for each group.

Available in $group stage only.

$range Outputs an array containing a sequence of integers according to user-defined inputs.
$reduce Applies an expression to each element in an array and combines them into a single value.
$reverseArray Returns an array with the elements in reverse order.
$second Returns the seconds for a date as a number between 0 and 60 (leap seconds).
$setDifference Returns a set with elements that appear in the first set but not in the second set; i.e. performs a relative complement of the second set relative to the first. Accepts exactly two argument expressions.
$setEquals Returns true if the input sets have the same distinct elements. Accepts two or more argument expressions.
$setIntersection Returns a set with elements that appear in all of the input sets. Accepts any number of argument expressions.
$setIsSubset Returns true if all elements of the first set appear in the second set, including when the first set equals the second set; i.e. not a strict subset. Accepts exactly two argument expressions.
$setUnion Returns a set with elements that appear in any of the input sets.
$size Returns the number of elements in the array. Accepts a single expression as argument.
$slice Returns a subset of an array.
$split Splits a string into substrings based on a delimiter. Returns an array of substrings. If the delimiter is not found within the string, returns an array containing the original string.
$sqrt Calculates the square root.
$stdDevPop

Returns the population standard deviation of the input values.

Changed in version 3.2: Available in both $group and $project stages.

$stdDevSamp

Returns the sample standard deviation of the input values.

Changed in version 3.2: Available in both $group and $project stages.

$strcasecmp Performs case-insensitive string comparison and returns: 0 if two strings are equivalent, 1 if the first string is greater than the second, and -1 if the first string is less than the second.
$strLenBytes Returns the number of UTF-8 encoded bytes in a string.
$strLenCP Returns the number of UTF-8 code points in a string.
$substr Deprecated. Use $substrBytes or $substrCP.
$substrBytes Returns the substring of a string. Starts with the character at the specified UTF-8 byte index (zero-based) in the string and continues for the specified number of bytes.
$substrCP Returns the substring of a string. Starts with the character at the specified UTF-8 code point (CP) index (zero-based) in the string and continues for the number of code points specified.
$subtract Returns the result of subtracting the second value from the first. If the two values are numbers, return the difference. If the two values are dates, return the difference in milliseconds. If the two values are a date and a number in milliseconds, return the resulting date. Accepts two argument expressions. If the two values are a date and a number, specify the date argument first as it is not meaningful to subtract a date from a number.
$sum

Returns a sum of numerical values. Ignores non-numeric values.

Changed in version 3.2: Available in both $group and $project stages.

$switch Evaluates a series of case expressions. When it finds an expression which evaluates to true, $switch executes a specified expression and breaks out of the control flow.
$toLower Converts a string to lowercase. Accepts a single argument expression.
$toUpper Converts a string to uppercase. Accepts a single argument expression.
$trunc Truncates a number to its integer.
$type Return the BSON data type of the field.
$week Returns the week number for a date as a number between 0 (the partial week that precedes the first Sunday of the year) and 53 (leap year).
$year Returns the year for a date as a number (e.g. 2014).
$zip Merge two arrays together.

For the pipeline stages, see Aggregation Pipeline Stages.