Aggregation Operators
On this page
- Compatibility
- Expression Operators
- Arithmetic Expression Operators
- Array Expression Operators
- Bitwise Operators
- Boolean Expression Operators
- Comparison Expression Operators
- Conditional Expression Operators
- Custom Aggregation Expression Operators
- Data Size Operators
- Date Expression Operators
- Literal Expression Operator
- Miscellaneous Operators
- Object Expression Operators
- Set Expression Operators
- String Expression Operators
- Text Expression Operator
- Timestamp Expression Operators
- Trigonometry Expression Operators
- Type Expression Operators
- Accumulators (
$group, $bucket, $bucketAuto, $setWindowFields
) - Accumulators (in Other Stages)
- Variable Expression Operators
- Window Operators
The aggregation pipeline operators are compatible with MongoDB Atlas and on-premise environments.
Note
For details on a specific operator, including syntax and examples, click on the link to the operator's reference page.
Compatibility
You can use the aggregation pipeline operators for deployments hosted in the following environments:
MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud
MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB
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 |
---|---|
Returns the absolute value of a number. | |
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. | |
Returns the smallest integer greater than or equal to the specified number. | |
Returns the result of dividing the first number by the
second. Accepts two argument expressions. | |
Raises e to the specified exponent. | |
Returns the largest integer less than or equal to the
specified number. | |
Calculates the natural log of a number. | |
Calculates the log of a number in the specified base. | |
Calculates the log base 10 of a number. | |
Returns the remainder of the first number divided by the
second. Accepts two argument expressions. | |
Multiplies numbers to return the product. Accepts any number
of argument expressions. | |
Raises a number to the specified exponent. | |
Rounds a number to to a whole integer or to a specified
decimal place. | |
Calculates the square root. | |
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. | |
Truncates a number to a whole integer or to a specified
decimal place. |
Array Expression Operators
Name | Description |
---|---|
Returns the element at the specified array index. | |
Converts an array of key value pairs to a document. | |
Concatenates arrays to return the concatenated array. | |
Selects a subset of the array to return an array with only
the elements that match the filter condition. | |
Returns a specified number of elements from the beginning of an array.
Distinct from the $firstN accumulator. | |
Returns a boolean indicating whether a specified value is in
an array. | |
Searches an array for an occurrence of a specified value and
returns the array index of the first occurrence. Array indexes start
at zero. | |
Determines if the operand is an array. Returns a boolean. | |
Returns a specified number of elements from the end of an array.
Distinct from the $lastN accumulator. | |
Applies a subexpression to each element of an array and
returns the array of resulting values in order. Accepts named
parameters. | |
Returns the n largest values in an array.
Distinct from the $maxN accumulator. | |
Returns the n smallest values in an array.
Distinct from the $minN accumulator. | |
Converts a document to an array of documents representing
key-value pairs. | |
Outputs an array containing a sequence of integers according
to user-defined inputs. | |
Applies an expression to each element in an array and
combines them into a single value. | |
Returns an array with the elements in reverse order. | |
Returns the number of elements in the array. Accepts a single
expression as argument. | |
Returns a subset of an array. | |
Sorts the elements of an array. | |
Merge two arrays together. |
Bitwise Operators
Name | Description |
---|---|
Returns the result of a bitwise New in version 6.3. | |
Returns the result of a bitwise New in version 6.3. | |
Returns the result of a bitwise New in version 6.3. | |
Returns the result of a bitwise New in version 6.3. |
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 |
---|---|
Returns true only when all its expressions evaluate to
true . Accepts any number of argument expressions. | |
Returns the boolean value that is the opposite of its
argument expression. Accepts a single argument expression. | |
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.
Name | Description |
---|---|
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. | |
Returns true if the values are equivalent. | |
Returns true if the first value is greater than the
second. | |
Returns true if the first value is greater than or equal
to the second. | |
Returns true if the first value is less than the second. | |
Returns true if the first value is less than or equal to
the second. | |
Returns true if the values are not equivalent. |
Conditional Expression Operators
Name | Description |
---|---|
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. | |
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. | |
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. |
Custom Aggregation Expression Operators
Name | Description |
---|---|
Defines a custom accumulator function. New in version 4.4. | |
Defines a custom function. New in version 4.4. |
Data Size Operators
The following operators return the size of a data element:
Name | Description |
---|---|
Returns the size of a given string or binary data value's
content in bytes. | |
Returns the size in bytes of a given document (i.e. bsontype
Object ) when encoded as BSON. |
Date Expression Operators
The following operators returns date objects or components of a date object:
Name | Description |
---|---|
Adds a number of time units to a date object. | |
Returns the difference between two dates. | |
Constructs a BSON Date object given the date's constituent
parts. | |
Converts a date/time string to a date object. | |
Subtracts a number of time units from a date object. | |
Returns a document containing the constituent parts of a date. | |
Returns the date as a formatted string. | |
Truncates a date. | |
Returns the day of the month for a date as a number between 1
and 31. | |
Returns the day of the week for a date as a number between 1
(Sunday) and 7 (Saturday). | |
Returns the day of the year for a date as a number between 1
and 366 (leap year). | |
Returns the hour for a date as a number between 0 and 23. | |
Returns the weekday number in ISO 8601 format, ranging from
1 (for Monday) to 7 (for Sunday). | |
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. | |
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). | |
Returns the milliseconds of a date as a number between 0 and
999. | |
Returns the minute for a date as a number between 0 and 59. | |
Returns the month for a date as a number between 1 (January)
and 12 (December). | |
Returns the seconds for a date as a number between 0 and 60
(leap seconds). | |
Converts value to a Date. New in version 4.0. | |
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). | |
Returns the year for a date as a number (e.g. 2014). |
The following arithmetic operators can take date operands:
Name | Description |
---|---|
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. | |
Returns the result of subtracting the second value from the
first. 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. |
Literal Expression Operator
Miscellaneous Operators
Name | Description |
---|---|
Returns the value of a specified field from a document.
You can use New in version 5.0. | |
Returns a random float between 0 and 1 | |
Randomly select documents at a given rate. Although the exact
number of documents selected varies on each run, the quantity
chosen approximates the sample rate expressed as a percentage
of the total number of documents. | |
Computes and returns the hash of the input expression using
the same hash function that MongoDB uses to create a hashed
index. |
Object Expression Operators
Name | Description |
---|---|
Combines multiple documents into a single document. | |
Converts a document to an array of documents representing
key-value pairs. | |
Adds, updates, or removes a specified field in a document. You
can use New in version 5.0. |
Set Expression Operators
Set expressions performs set operation on arrays, treating arrays as sets. Set expressions ignores the duplicate entries in each input array and the order of the elements.
If the set operation returns a set, the operation filters out duplicates in the result to output an array that contains only unique entries. The order of the elements in the output array is unspecified.
If a set contains a nested array element, the set expression does not descend into the nested array but evaluates the array at top-level.
Name | Description |
---|---|
Returns true if no element of a set evaluates to
false , otherwise, returns false . Accepts a single
argument expression. | |
Returns true if any elements of a set evaluate to
true ; otherwise, returns false . Accepts a single
argument expression. | |
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. | |
Returns true if the input sets have the same distinct
elements. Accepts two or more argument expressions. | |
Returns a set with elements that appear in all of the
input sets. Accepts any number of argument expressions. | |
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. | |
Returns a set with elements that appear in any of the
input sets. |
String Expression Operators
String expressions, with the exception of
$concat
, only have a well-defined behavior for strings of ASCII characters.
$concat
behavior is well-defined regardless of
the characters used.
Name | Description |
---|---|
Concatenates any number of strings. | |
Converts a date/time string to a date object. | |
Returns the date as a formatted string. | |
Searches a string for an occurrence of a substring and returns
the UTF-8 byte index of the first occurrence. If the substring
is not found, returns -1 . | |
Searches a string for an occurrence of a substring and returns
the UTF-8 code point index of the first occurrence. If the
substring is not found, returns -1 | |
Removes whitespace or the specified characters from the beginning of a string. New in version 4.0. | |
Applies a regular expression (regex) to a string and returns information on the first matched substring. New in version 4.2. | |
Applies a regular expression (regex) to a string and returns information on the all matched substrings. New in version 4.2. | |
Applies a regular expression (regex) to a string and returns a boolean that indicates if a match is found or not. New in version 4.2. | |
Replaces the first instance of a matched string in a given input. New in version 4.4. | |
Replaces all instances of a matched string in a given input. New in version 4.4. | |
Removes whitespace or the specified characters from the end of a string. New in version 4.0. | |
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. | |
Returns the number of UTF-8 encoded bytes in a string. | |
Returns the number of UTF-8 code points in a string. | |
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. | |
Deprecated. Use $substrBytes or
$substrCP . | |
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. | |
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. | |
Converts a string to lowercase. Accepts a single argument
expression. | |
Converts value to a string. New in version 4.0. | |
Removes whitespace or the specified characters from the beginning and end of a string. New in version 4.0. | |
Converts a string to uppercase. Accepts a single argument
expression. |
Text Expression Operator
Name | Description |
---|---|
Access available per-document metadata related to the
aggregation operation. |
Timestamp Expression Operators
Timestamp expression operators return values from a timestamp.
Trigonometry Expression Operators
Trigonometry expressions perform trigonometric operations on numbers.
Values that represent angles are always input or output in radians. Use
$degreesToRadians
and $radiansToDegrees
to
convert between degree and radian measurements.
Name | Description |
---|---|
Returns the sine of a value that is measured in radians. | |
Returns the cosine of a value that is measured in radians. | |
Returns the tangent of a value that is measured in radians. | |
Returns the inverse sin (arc sine) of a value in radians. | |
Returns the inverse cosine (arc cosine) of a value in radians. | |
Returns the inverse tangent (arc tangent) of a value in
radians. | |
Returns the inverse tangent (arc tangent) of y / x in
radians, where y and x are the first and second
values passed to the expression respectively. | |
Returns the inverse hyperbolic sine (hyperbolic arc sine) of a
value in radians. | |
Returns the inverse hyperbolic cosine (hyperbolic arc cosine)
of a value in radians. | |
Returns the inverse hyperbolic tangent (hyperbolic arc
tangent) of a value in radians. | |
Returns the hyperbolic sine of a value that is measured in
radians. | |
Returns the hyperbolic cosine of a value that is measured in
radians. | |
Returns the hyperbolic tangent of a value that is measured in
radians. | |
Converts a value from degrees to radians. | |
Converts a value from radians to degrees. |
Type Expression Operators
Name | Description |
---|---|
Converts a value to a specified type. New in version 4.0. | |
Converts value to a boolean. New in version 4.0. | |
Converts value to a Date. New in version 4.0. | |
Converts value to a Decimal128. New in version 4.0. | |
Converts value to a double. New in version 4.0. | |
Converts value to an integer. New in version 4.0. | |
Converts value to a long. New in version 4.0. | |
Converts value to an ObjectId. New in version 4.0. | |
Converts value to a string. New in version 4.0. | |
Return the BSON data type of the field. |
Accumulators ($group, $bucket, $bucketAuto, $setWindowFields
)
Aggregation accumulator operators:
Maintain their state as documents progress through the aggregation pipeline.
Return totals, maxima, minima, and other values.
Can be used in these aggregation pipeline stages:
$setWindowFields
starting in MongoDB 5.0 (except when you are using the$accumulator
or$mergeObjects
operators, which cannot be used with$setWindowFields
)
Changed in version 5.0.
Name | Description |
---|---|
Returns the result of a user-defined accumulator function. | |
Returns an array of unique expression values for each group. Order of the array elements is undefined. Changed in version 5.0: Available in the | |
Returns an average of numerical values. Ignores non-numeric values. Changed in version 5.0: Available in the | |
Returns the bottom element within a group according to the specified sort order. New in version 5.2. Available in the | |
Returns an aggregation of the bottom New in version 5.2. Available in the | |
Returns the number of documents in a group. Distinct from the New in version 5.0: Available in the | |
Returns the result of an expression for the first document in a group. Changed in version 5.0: Available in the | |
Returns an aggregation of the first New in version 5.2: Available in the | |
Returns the result of an expression for the last document in a group. Changed in version 5.0: Available in the | |
Returns an aggregation of the last New in version 5.2: Available in the | |
Returns the highest expression value for each group. Changed in version 5.0: Available in the | |
Returns an aggregation of the New in version 5.2. Available in | |
Returns an approximation of the median, the 50th percentile, as a scalar value. New in version 7.0. This operator is available as an accumulator in these stages: It is also available as an aggregation expression. | |
Returns a document created by combining the input documents
for each group. | |
Returns the lowest expression value for each group. Changed in version 5.0: Available in the | |
Returns an array of scalar values that correspond to specified percentile values. New in version 7.0. This operator is available as an accumulator in these stages: It is also available as an aggregation expression. | |
Returns an array of expression values for documents in each group. Changed in version 5.0: Available in the | |
Returns the population standard deviation of the input values. Changed in version 5.0: Available in the | |
Returns the sample standard deviation of the input values. Changed in version 5.0: Available in the | |
Returns a sum of numerical values. Ignores non-numeric values. Changed in version 5.0: Available in the | |
Returns the top element within a group according to the specified sort order. New in version 5.2. Available in the | |
Returns an aggregation of the top New in version 5.2. Available in the |
Accumulators (in Other Stages)
Some operators that are available as accumulators for the
$group
stage are also available for use in other stages but
not as accumulators. When used in these other stages, these operators
do not maintain their state and can take as input either a single
argument or multiple arguments. For details, refer to the specific
operator page.
Changed in version 5.0.
The following accumulator operators are also available in the
$project
, $addFields
, $set
, and,
starting in MongoDB 5.0, the $setWindowFields
stages.
Name | Description |
---|---|
Returns an average of the specified expression or list of
expressions for each document. Ignores non-numeric values. | |
Returns the result of an expression for the first document in a group. | |
Returns the result of an expression for the last document in a group. | |
Returns the maximum of the specified expression or list of
expressions for each document | |
Returns an approximation of the median, the 50th percentile, as a scalar value. New in version 7.0. This operator is available as an accumulator in these stages: It is also available as an aggregation expression. | |
Returns the minimum of the specified expression or list of
expressions for each document | |
Returns an array of scalar values that correspond to specified percentile values. New in version 7.0. This operator is available as an accumulator in these stages: It is also available as an aggregation expression. | |
Returns the population standard deviation of the input values. | |
Returns the sample standard deviation of the input values. | |
Returns a sum of numerical values. Ignores non-numeric values. |
Variable Expression Operators
Name | Description |
---|---|
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. |
Window Operators
New in version 5.0.
Window operators return values from a defined span of documents from
a collection, known as a window. A window is defined in the
$setWindowFields
stage, available starting in MongoDB
5.0.
The following window operators are available in the
$setWindowFields
stage.
Name | Description |
---|---|
Returns an array of all unique values that results from applying an expression to each document. Changed in version 5.0: Available in the | |
Returns the average for the specified expression. Ignores non-numeric values. Changed in version 5.0: Available in the | |
Returns the bottom element within a group according to the specified sort order. New in version 5.2. Available in the | |
Returns an aggregation of the bottom New in version 5.2. Available in the | |
Returns the number of documents in the group or window. Distinct from the New in version 5.0. | |
Returns the population covariance of two numeric expressions. New in version 5.0. | |
Returns the sample covariance of two numeric expressions. New in version 5.0. | |
Returns the document position (known as the rank) relative to
other documents in the New in version 5.0. | |
Returns the average rate of change within the specified window. New in version 5.0. | |
Returns the position of a document (known as the document
number) in the New in version 5.0. | |
Returns the exponential moving average for the numeric expression. New in version 5.0. | |
Returns the result of an expression for the first document in a group or window. Changed in version 5.0: Available in the | |
Returns the approximation of the area under a curve. New in version 5.0. | |
Returns the result of an expression for the last document in a group or window. Changed in version 5.0: Available in the | |
Fills Available in the New in version 5.3. | |
Last observation carried forward. Sets values for Available in the New in version 5.2. | |
Returns the maximum value that results from applying an expression to each document. Changed in version 5.0: Available in the | |
Returns the minimum value that results from applying an expression to each document. Changed in version 5.0: Available in the | |
Returns an aggregation of the New in version 5.2. Available in | |
Returns an array of values that result from applying an expression to each document. Changed in version 5.0: Available in the | |
Returns the document position (known as the rank) relative to
other documents in the New in version 5.0. | |
Returns the value from an expression applied to a document in a
specified position relative to the current document in the
New in version 5.0. | |
Returns the population standard deviation that results from applying a numeric expression to each document. Changed in version 5.0: Available in the | |
Returns the sample standard deviation that results from applying a numeric expression to each document. Changed in version 5.0: Available in the | |
Returns the sum that results from applying a numeric expression to each document. Changed in version 5.0: Available in the | |
Returns the top element within a group according to the specified sort order. New in version 5.2. Available in the | |
Returns an aggregation of the top New in version 5.2. Available in the |
For the pipeline stages, see Aggregation Stages.