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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.

Arithmetic Expression Operators

Arithmetic expressions perform mathematic operations on numbers. Some arithmetic expressions can also support date arithmetic.

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.
$ceil Returns the smallest integer greater than or equal to the specified number.
$divide Returns the result of dividing the first number by the second. Accepts two argument expressions.
$exp Raises e to the specified exponent.
$floor Returns the largest integer less than or equal to the specified number.
$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.
$mod Returns the remainder of the first number divided by the second. Accepts two argument expressions.
$multiply Multiplies numbers to return the product. Accepts any number of argument expressions.
$pow Raises a number to the specified exponent.
$sqrt Calculates the square root.
$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.
$trunc Truncates a number to its integer.

Array Expression Operators

$arrayElemAt Returns the element at the specified array index.
$arrayToObject Converts an array of key value pairs to a document.
$concatArrays Concatenates arrays to return the concatenated array.
$filter Selects a subset of the array to return an array with only the elements that match the filter condition.
$in Returns a boolean indicating whether a specified value is in an array.
$indexOfArray Searches an array for an occurrence of a specified value and returns the array index of the first occurrence. If the substring is not found, returns -1.
$isArray Determines if the operand is an array. Returns a boolean.
$map Applies a subexpression to each element of an array and returns the array of resulting values in order. Accepts named parameters.
$objectToArray Converts a document to an array of documents representing key-value pairs.
$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.
$size Returns the number of elements in the array. Accepts a single expression as argument.
$slice Returns a subset of an array.
$zip Merge two arrays together.

Boolean Expression Operators

Boolean expressions evaluate their argument expressions as booleans and return a boolean as the result.

In addition to the false boolean value, Boolean expression evaluates as false the following: null, 0, and undefined values. The Boolean expression evaluates all other values as true, including non-zero numeric values and arrays.

Name Description
$and Returns true only when all its expressions evaluate to true. Accepts any number of argument expressions.
$not Returns the boolean value that is the opposite of its argument expression. Accepts a single argument expression.
$or Returns true when any of its expressions evaluates to true. Accepts any number of argument expressions.

Comparison Expression Operators

Comparison expressions return a boolean except for $cmp which returns a number.

The comparison expressions take two argument expressions and compare both value and type, using the specified BSON comparison order for values of different types.

$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.
$eq Returns true if the values are equivalent.
$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.
$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.
$ne Returns true if the values are not equivalent.

Conditional Expression Operators

Name Description
$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.
$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.
$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.

Date Expression Operators

The following operators returns date objects or components of a date object:

Name Description
$dateFromParts Constructs a BSON Date object given the date’s constituent parts.
$dateFromString Converts a date/time string to a date object.
$dateToParts Returns a document containing the constituent parts of a date.
$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).
$hour Returns the hour for a date as a number between 0 and 23.
$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).
$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.
$month Returns the month for a date as a number between 1 (January) and 12 (December).
$second Returns the seconds for a date as a number between 0 and 60 (leap seconds).
$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).

The following arithmetic operators can take date operands:

Name Description
$add 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.