Aggregation Pipeline Quick Reference聚合管道快速参考

Stages阶段 (db.collection.aggregate)

In the db.collection.aggregate() method, pipeline stages appear in an array. 在db.collection.aggregate()方法中，管道阶段显示在一个数组中。Documents pass through the stages in sequence. 文件按顺序通过各个阶段。All except the $out, $merge, and $geoNear stages can appear multiple times in a pipeline.除$out、$merge和$geoNear阶段外，所有阶段都可以在管道中多次出现。

db.collection.aggregate( [ { <stage> }, ... ] )

Stages阶段 (db.aggregate)

Starting in version 3.6, MongoDB also provides the db.aggregate() method:从3.6版本开始，MongoDB还提供了db.aggregate()方法：

db.aggregate( [ { <stage> }, ... ] )

The following stages use the db.aggregate() method and not the db.collection.aggregate() method.以下阶段使用db.aggregate()方法，而不是db.collection.aggregate()方法。

Stages Available for Updates可用于更新的阶段

Starting in MongoDB 4.2, you can use the aggregation pipeline for updates in:从MongoDB 4.2开始，您可以在以下位置使用聚合管道进行更新：

For the updates, the pipeline can consist of the following stages:对于更新，管道可以由以下阶段组成：

• $addFields and its alias 及其别名$set
• $project and its alias 及其别名$unset
• $replaceRoot and its alias 及其别名$replaceWith.

Field Paths字段路径

Aggregation expressions use field path to access fields in the input documents. 聚合表达式使用字段路径访问输入文档中的字段。To specify a field path, prefix the field name or the dotted field name (if the field is in the embedded document) with a dollar sign $. 若要指定字段路径，请在字段名或虚线字段名（如果字段在嵌入文档中）前面加上美元符号$。For example, "$user" to specify the field path for the user field or "$user.name" to specify the field path to "user.name" field.例如，"$user"指定user字段的字段路径，或"$user.name"指定"user.name"字段的字段通路。

"$<field>" is equivalent to "$$CURRENT.<field>" where the CURRENT is a system variable that defaults to the root of the current object, unless stated otherwise in specific stages."$<field>"相当于"$$CURRENT.<field>"，其中CURRENT是一个系统变量，默认为当前对象的根，除非在特定阶段另有说明。

Aggregation Variables聚合变量

MongoDB provides various aggregation system variables for use in expressions. MongoDB提供了用于表达式的各种聚合系统变量。To access variables, prefix the variable name with $$. 要访问变量，请在变量名前面加上$$。For example:例如：

For a more detailed description of these variables, see system variables.有关这些变量的更详细描述，请参阅系统变量。

Literals字面量

Literals can be of any type. 字面量可以是任何类型。However, MongoDB parses string literals that start with a dollar sign $ as a path to a field and numeric/boolean literals in expression objects as projection flags. 然而，MongoDB解析以美元符号$开头的字符串字面量作为字段的路径，解析表达式对象中的数字/布尔字面量作为投影标志。To avoid parsing literals, use the $literal expression.为了避免解析字面量，请使用$literal表达式。

Expression Objects表达式对象

Expression objects have the following form:表达式对象具有以下形式：

{ <field1>: <expression1>, ... }

If the expressions are numeric or boolean literals, MongoDB treats the literals as projection flags (e.g. 1 or true to include the field), valid only in the $project stage. 如果表达式是数字或布尔文本，MongoDB将文本视为投影标志（例如，1或true以包含字段），仅在$project阶段有效。To avoid treating numeric or boolean literals as projection flags, use the $literal expression to wrap the numeric or boolean literals.若要避免将数字或布尔字面量视为投影标志，请使用$literal表达式包装数字或布尔文本。

Arithmetic Expression Operators算术表达式运算符

Arithmetic expressions perform mathematic operations on numbers. 算术表达式对数字进行数学运算。Some arithmetic expressions can also support date arithmetic.一些算术表达式也可以支持日期算术。

Array Expression Operators数组表达式运算符

Boolean Expression Operators布尔表达式运算符

Boolean expressions evaluate their argument expressions as booleans and return a boolean as the result.布尔表达式将其参数表达式计算为布尔值，并返回布尔值作为结果。

In addition to the false boolean value, Boolean expression evaluates as false the following: null, 0, and undefined values. 除了false布尔值之外，布尔表达式还将以下值计算为false：null、0和undefined值。The Boolean expression evaluates all other values as true, including non-zero numeric values and arrays.布尔表达式将所有其他值计算为true，包括非零数值和数组。

Comparison Expression Operators比较表达式运算符

Comparison expressions return a boolean except for $cmp which returns a number.比较表达式返回一个布尔值，但$cmp返回一个数字。

The comparison expressions take two argument expressions and compare both value and type, using the specified BSON comparison order for values of different types.比较表达式采用两个参数表达式，并对不同类型的值使用指定的BSON比较顺序来比较值和类型。

Conditional Expression Operators条件表达式运算符

Custom Aggregation Expression Operators自定义聚合表达式运算符

Data Size Expression Operators数据大小表达式运算符

The following operators return the size of a data element:以下运算符返回数据元素的大小：

Date Expression Operators日期表达式运算符

The following operators returns date objects or components of a date object:以下运算符返回日期对象或日期对象的组件：

The following arithmetic operators can take date operands:以下算术运算符可以接受日期操作数：

Literal Expression Operator文字表达式运算符

Miscellaneous Operators混杂运算符

Object Expression Operators对象表达式运算符

Set Expression Operators设置表达式运算符

Set expressions performs set operation on arrays, treating arrays as sets. 集合表达式对数组执行集合运算，将数组视为集合。Set expressions ignores the duplicate entries in each input array and the order of the elements.Set表达式会忽略每个输入数组中的重复项和元素的顺序。

If the set operation returns a set, the operation filters out duplicates in the result to output an array that contains only unique entries. 如果set操作返回一个集合，则该操作会筛选出结果中的重复项，以输出仅包含唯一项的数组。The order of the elements in the output array is unspecified.输出数组中元素的顺序未指定。

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.如果集合包含嵌套数组元素，则集合表达式不会下降到嵌套数组中，而是在顶级计算数组。

Name名称	Description描述
$allElementsTrue	Returns true if no element of a set evaluates to false, otherwise, returns false. 如果集合中没有任何元素的计算结果为false，则返回true，否则返回false。Accepts a single argument expression.接受单个参数表达式。
$anyElementTrue	Returns true if any elements of a set evaluate to true; otherwise, returns false. 如果集合的任何元素的求值结果为true，则返回true；否则，返回false。Accepts a single argument expression.接受单个参数表达式。
$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. 返回元素出现在第一个集合中但不出现在第二个集合中的集合；即执行第二集合相对于第一集合的相对补码。Accepts exactly two argument expressions.正好接受两个参数表达式。
$setEquals	Returns true if the input sets have the same distinct elements. 如果输入集具有相同的不同元素，则返回true。Accepts two or more argument expressions.接受两个或多个参数表达式。
$setIntersection	Returns a set with elements that appear in all of the input sets. 返回包含出现在所有输入集中的元素的集合。Accepts any number of argument expressions.接受任意数量的参数表达式。
$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. 如果第一个集合的所有元素都出现在第二个集合中，包括当第一个集合等于第二个集时，返回true；即不是严格的子集。Accepts exactly two argument expressions.正好接受两个参数表达式。
$setUnion	Returns a set with elements that appear in any of the input sets.返回一个集，该集包含出现在任何输入集中的元素。

String Expression Operators字符串表达式运算符

String expressions, with the exception of $concat, only have a well-defined behavior for strings of ASCII characters.字符串表达式（$concat除外）只对ASCII字符字符串具有定义良好的行为。

$concat behavior is well-defined regardless of the characters used.无论使用什么字符，行为都是定义良好的。

Name名称	Description描述
$concat	Concatenates any number of strings.连接任意数量的字符串。
$dateFromString	Converts a date/time string to a date object.将日期/时间字符串转换为日期对象。
$dateToString	Returns the date as a formatted string.以格式化字符串形式返回日期。
$indexOfBytes	Searches a string for an occurrence of a substring and returns the UTF-8 byte index of the first occurrence. 在字符串中搜索子字符串的出现，并返回第一个出现的UTF-8字节索引。If the substring is not found, returns -1.如果未找到子字符串，则返回-1。
$indexOfCP	Searches a string for an occurrence of a substring and returns the UTF-8 code point index of the first occurrence. 在字符串中搜索子字符串的出现，并返回第一个出现的UTF-8代码点索引。If the substring is not found, returns -1如果未找到子字符串，则返回-1
$ltrim	Removes whitespace or the specified characters from the beginning of a string. 删除字符串开头的空白或指定字符。 New in version 4.0. 4.0版新增。
$regexFind	Applies a regular expression (regex) to a string and returns information on the first matched substring. 将正则表达式（regex）应用于字符串，并返回第一个匹配子字符串的信息。 New in version 4.2. 4.2版新增。
$regexFindAll	Applies a regular expression (regex) to a string and returns information on the all matched substrings. 将正则表达式（regex）应用于字符串，并返回所有匹配子字符串的信息。 New in version 4.2. 4.2版新增。
$regexMatch	Applies a regular expression (regex) to a string and returns a boolean that indicates if a match is found or not. 将正则表达式（regex）应用于字符串，并返回一个布尔值，指示是否找到匹配项。 New in version 4.2. 4.2版新增。
$replaceOne	Replaces the first instance of a matched string in a given input. 替换给定输入中匹配字符串的第一个实例。 New in version 4.4. 4.4版新增。
$replaceAll	Replaces all instances of a matched string in a given input. 替换给定输入中匹配字符串的所有实例。 New in version 4.4. 4.4版新增。
$rtrim	Removes whitespace or the specified characters from the end of a string. 删除字符串末尾的空白或指定字符。 New in version 4.0. 4.0版新增。
$split	Splits a string into substrings based on a delimiter. 根据分隔符将字符串拆分为子字符串。Returns an array of substrings. 返回一个子字符串数组。If the delimiter is not found within the string, returns an array containing the original string.如果在字符串中找不到分隔符，则返回包含原始字符串的数组。
$strLenBytes	Returns the number of UTF-8 encoded bytes in a string.返回字符串中UTF-8编码的字节数。
$strLenCP	Returns the number of UTF-8 code points in a string.返回字符串中UTF-8代码点的数目。
$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.执行不区分大小写的字符串比较并返回：如果两个字符串相等，则返回0；如果第一个字符串大于第二个字符串，则返回1；如果第一条字符串小于第二个，则返回-1。
$substr	Deprecated. 已弃用。Use $substrBytes or $substrCP.请改用$substrBytes或$substrCP。
$substrBytes	Returns the substring of a string. 返回字符串的子字符串。Starts with the character at the specified UTF-8 byte index (zero-based) in the string and continues for the specified number of bytes.从字符串中指定的UTF-8字节索引（从零开始）处的字符开始，并持续指定的字节数。
$substrCP	Returns the substring of a string. 返回字符串的子字符串。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.以字符串中指定的UTF-8代码点（CP）索引（从零开始）处的字符开头，并按指定的代码点数量继续。
$toLower	Converts a string to lowercase. Accepts a single argument expression.将字符串转换为小写。接受单个参数表达式。
$toString	Converts value to a string. 将值转换为字符串。 New in version 4.0. 4.0版新增。
$trim	Removes whitespace or the specified characters from the beginning and end of a string. 删除字符串开头和结尾的空白或指定字符。 New in version 4.0. 4.0版新增。
$toUpper	Converts a string to uppercase. 将字符串转换为大写。Accepts a single argument expression.接受单个参数表达式。

Text Expression Operator文本表达式运算符

Trigonometry Expression Operators三角表达式运算符

Trigonometry expressions perform trigonometric operations on numbers. 三角表达式对数字执行三角运算。Values that represent angles are always input or output in radians.表示角度的值始终以弧度为单位输入或输出。 Use $degreesToRadians and $radiansToDegrees to convert between degree and radian measurements.使用$degreesToRadians和$radiansToDegrees在度数和弧度测量之间转换。

Type Expression Operators类型表达式运算符

Accumulators 累加器($group, $bucket, $bucketAuto, $setWindowFields)

Aggregation accumulator operators:聚合累加器运算符：

• Maintain their state as documents progress through the aggregation pipeline.在文档通过聚合管道时维护它们的状态。
• Return totals, maxima, minima, and other values.返回总计、最大值、最小值和其他值。
• Can be used in these aggregation pipeline stages:可用于以下聚合管道阶段：
  • $bucket
  • $bucketAuto
  • $group
  • $setWindowFields starting in MongoDB 5.0 (except when you are using the $accumulator or $mergeObjects operators, which cannot be used with $setWindowFields)从MongoDB 5.0开始（除非使用$accumulator或$mergeObjects运算符，它们不能与$setWindowFields一起使用）

Accumulators (in Other Stages)蓄能器（其他阶段）

Some operators that are available as accumulators for the $group stage are also available for use in other stages but not as accumulators. 一些可作为$group阶段的累加器的运算符也可用于其他阶段，但不能用作累加器。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.当在这些其他阶段中使用时，这些运算符不维护其状态，可以将单个参数或多个参数作为输入。有关详细信息，请参阅特定的运算符页面。

The following accumulator operators are also available in the $project, $addFields, $set, and, starting in MongoDB 5.0, the $setWindowFields stages.以下累加器运算符也可用于$project、$addFields、$set，以及从MongoDB 5.0开始的$setWindowFields阶段。

Variable Expression Operators变量表达式运算符

Window Operators窗口运算符

Window operators return values from a defined span of documents from a collection, known as a window. 窗口运算符从集合中定义的文档范围（称为窗口）中返回值。A window is defined in the $setWindowFields stage, available starting in MongoDB 5.0.窗口是在$setWindowFields 阶段定义的，从MongoDB 5.0开始就可以使用。

The following window operators are available in the $setWindowFields stage.以下窗口运算符在$setWindowFields阶段中可用。

Name名称	Description描述
$addToSet	Returns an array of all unique values that results from applying an expression to each document. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$avg	Returns the average for the specified expression. Ignores non-numeric values. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$bottom	Returns the bottom element within a group according to the specified sort order. New in version 5.2. 5.2版新增。 Available in the $group and $setWindowFields stages.
$bottomN	Returns an aggregation of the bottom n fields within a group, according to the specified sort order. New in version 5.2. 5.2版新增。 Available in the $group and $setWindowFields stages.
$count	Returns the number of documents in the group or window. Distinct from the $count pipeline stage. New in version 5.0. 5.0版新增。
$covariancePop	Returns the population covariance of two numeric expressions. New in version 5.0. 5.0版新增。
$covarianceSamp	Returns the sample covariance of two numeric expressions. New in version 5.0. 5.0版新增。
$denseRank	Returns the document position (known as the rank) relative to other documents in the $setWindowFields stage partition. There are no gaps in the ranks. Ties receive the same rank. New in version 5.0. 5.0版新增。
$derivative	Returns the average rate of change within the specified window. New in version 5.0. 5.0版新增。
$documentNumber	Returns the position of a document (known as the document number) in the $setWindowFields stage partition. Ties result in different adjacent document numbers. New in version 5.0. 5.0版新增。
$expMovingAvg	Returns the exponential moving average for the numeric expression. New in version 5.0. 5.0版新增。
$first	Returns the result of an expression for the first document in a group or window. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$integral	Returns the approximation of the area under a curve. New in version 5.0. 5.0版新增。
$last	Returns the result of an expression for the last document in a group or window. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$linearFill	Fills null and missing fields in a window using linear interpolation based on surrounding field values. Available in the $setWindowFields stage. New in version 5.3. 5.3版新增。
$locf	Last observation carried forward. Sets values for null and missing fields in a window to the last non-null value for the field. Available in the $setWindowFields stage. New in version 5.2. 5.2版新增。
$max	Returns the maximum value that results from applying an expression to each document. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$min	Returns the minimum value that results from applying an expression to each document. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$minN	Returns an aggregation of the n minimum valued elements in a group. Distinct from the $minN array operator. New in version 5.2. 5.2版新增。 Available in $group, $setWindowFields and as an expression.
$push	Returns an array of values that result from applying an expression to each document. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$rank	Returns the document position (known as the rank) relative to other documents in the $setWindowFields stage partition. New in version 5.0. 5.0版新增。
$shift	Returns the value from an expression applied to a document in a specified position relative to the current document in the $setWindowFields stage partition. New in version 5.0. 5.0版新增。
$stdDevPop	Returns the population standard deviation that results from applying a numeric expression to each document. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$stdDevSamp	Returns the sample standard deviation that results from applying a numeric expression to each document. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$sum	Returns the sum that results from applying a numeric expression to each document. Changed in version 5.05.0版更改: Available in the $setWindowFields stage.
$top	Returns the top element within a group according to the specified sort order. New in version 5.2. 5.2版新增。 Available in the $group and $setWindowFields stages.
$topN	Returns an aggregation of the top n fields within a group, according to the specified sort order. New in version 5.2. 5.2版新增。 Available in the $group and $setWindowFields stages.