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聚合管道快速指南

聚合管道快速指南¶

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阶段
Expressions
Accumulators

注解

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

阶段¶

In the db.collection.aggregate method, pipeline stages appear in an array. Documents pass through the stages in sequence. All except the $out and $geoNear stages can appear multiple times in a pipeline.

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

Name	描述
`$collStats`	Returns statistics regarding a collection or view.
`$project`	修改流中的文档，比如增加或者移除一个字段。对于每一个输入文档，相应输出一个文档。
`$match`	对流中的文档进行过滤，仅允许符合条件的文档进入下一个阶段，过滤操作不会修改文档。 `$match` 操作使用Mongodb标准的查询条件。对于每一个输入文档，如果符合条件则输出这个文档，否则就丢弃该文档。
`$redact`	Reshapes each document in the stream by restricting the content for each document based on information stored in the documents themselves. Incorporates the functionality of `$project` and `$match`. Can be used to implement field level redaction. For each input document, outputs either one or zero documents.
`$limit`	Passes the first n documents unmodified to the pipeline where n is the specified limit. For each input document, outputs either one document (for the first n documents) or zero documents (after the first n documents).
`$skip`	Skips the first n documents where n is the specified skip number and passes the remaining documents unmodified to the pipeline. For each input document, outputs either zero documents (for the first n documents) or one document (if after the first n documents).
`$unwind`	Deconstructs an array field from the input documents to output a document for each element. Each output document replaces the array with an element value. For each input document, outputs n documents where n is the number of array elements and can be zero for an empty array.
`$group`	Groups input documents by a specified identifier expression and applies the accumulator expression(s), if specified, to each group. Consumes all input documents and outputs one document per each distinct group. The output documents only contain the identifier field and, if specified, accumulated fields.
`$sample`	Randomly selects the specified number of documents from its input.
`$sort`	Reorders the document stream by a specified sort key. Only the order changes; the documents remain unmodified. For each input document, outputs one document.
`$geoNear`	Returns an ordered stream of documents based on the proximity to a geospatial point. Incorporates the functionality of `$match`, `$sort`, and `$limit` for geospatial data. The output documents include an additional distance field and can include a location identifier field.
`$lookup`	Performs a left outer join to another collection in the same database to filter in documents from the “joined” collection for processing.
`$out`	Writes the resulting documents of the aggregation pipeline to a collection. To use the `$out` stage, it must be the last stage in the pipeline.
`$indexStats`	Returns statistics regarding the use of each index for the collection.
`$facet`	Processes multiple aggregation pipelines within a single stage on the same set of input documents. Enables the creation of multi-faceted aggregations capable of characterizing data across multiple dimensions, or facets, in a single stage.
`$bucket`	Categorizes incoming documents into groups, called buckets, based on a specified expression and bucket boundaries.
`$bucketAuto`	Categorizes incoming documents into a specific number of groups, called buckets, based on a specified expression. Bucket boundaries are automatically determined in an attempt to evenly distribute the documents into the specified number of buckets.
`$sortByCount`	Groups incoming documents based on the value of a specified expression, then computes the count of documents in each distinct group.
`$addFields`	Adds new fields to documents. Outputs documents that contain all existing fields from the input documents and newly added fields.
`$replaceRoot`	Replaces a document with the specified embedded document. The operation replaces all existing fields in the input document, including the `_id` field. Specify a document embedded in the input document to promote the embedded document to the top level.
`$count`	Returns a count of the number of documents at this stage of the aggregation pipeline.
`$graphLookup`	Performs a recursive search on a collection. To each output document, adds a new array field that contains the traversal results of the recursive search for that document.

Expressions¶

Expressions can include field paths and system variables, literals, expression objects, and expression operators. Expressions can be nested.

Field Path and System Variables¶

Aggregation expressions use field path to access fields in the input documents. To specify a field path, use a string that prefixes with a dollar sign $ the field name or the dotted field name, if the field is in embedded document. For example, "$user" to specify the field path for the user field or "$user.name" to specify the field path to "user.name" field.

"$<field>" is equivalent to "$$CURRENT.<field>" where the CURRENT is a system variable that defaults to the root of the current object in the most stages, unless stated otherwise in specific stages. CURRENT can be rebound.

Along with the CURRENT system variable, other system variables are also available for use in expressions. To use user-defined variables, use $let and $map expressions. To access variables in expressions, use a string that prefixes the variable name with $$.

Literals¶

Literals can be of any type. 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. To avoid parsing literals, use the $literal expression.

Expression Objects¶

Expression objects have the following form:

{ <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. To avoid treating numeric or boolean literals as projection flags, use the $literal expression to wrap the numeric or boolean literals.

Operator Expressions¶

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.

Boolean Expressions¶

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	描述
`$and`	Returns `true` only when all its expressions evaluate to `true`. Accepts any number of argument expressions.
`$or`	Returns `true` when any of its expressions evaluates 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.

Set Expressions¶

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	描述
`$setEquals`	Returns `true` if the input sets have the same distinct elements. Accepts two or more argument expressions.
`$setIntersection`	Returns a set with elements that appear in all of the input sets. Accepts any number of argument expressions.
`$setUnion`	Returns a set with elements that appear in any of the input sets. Accepts any number of argument expressions.
`$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.
`$setIsSubset`	Returns `true` if all elements of the first set appear in the second set, including when the first set equals the second set; i.e. not a strict subset. Accepts exactly two argument expressions.
`$anyElementTrue`	Returns `true` if any elements of a set evaluate to `true`; otherwise, returns `false`. Accepts a single argument expression.
`$allElementsTrue`	Returns `true` if no element of a set evaluates to `false`, otherwise, returns `false`. Accepts a single argument expression.

Comparison Expressions¶

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	描述
`$cmp`	Returns: `0` if the two values are equivalent, `1` if the first value is greater than the second, and `-1` if the first value is less than the second.
`$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.

Arithmetic Expressions¶

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

Name	描述
`$abs`	Returns the absolute value of a number.
`$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.

String Expressions¶

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	描述
`$concat`	Concatenates any number of strings.
`$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`.
`$indexOfCP`	Searches a string for an occurence of a substring and returns the UTF-8 code point index of the first occurence. If the substring is not found, returns `-1`.
`$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.
`$strcasecmp`	Performs case-insensitive string comparison and returns: `0` if two strings are equivalent, `1` if the first string is greater than the second, and `-1` if the first string is less than the second.
`$strLenBytes`	Returns the number of UTF-8 encoded bytes in a string.
`$strLenCP`	Returns the number of UTF-8 code points in a string.
`$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.
`$substr`	Deprecated. Use `$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.
`$substrCP`	Returns the substring of a string. Starts with the character at the specified UTF-8 code point (CP) index (zero-based) in the string and continues for the number of code points specified.
`$toLower`	Converts a string to lowercase. Accepts a single argument expression.
`$toUpper`	Converts a string to uppercase. Accepts a single argument expression.

Text Search Expressions¶

Name	描述
`$meta`	Access text search metadata.

Array Expressions¶

Name	描述
`$arrayElemAt`	Returns the element at the specified array index.
`$concatArrays`	Concatenates arrays to return the concatenated array.
`$filter`	Selects a subset of the array to return an array with only the elements that match the filter condition.
`$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`.
`$isArray`	Determines if the operand is an array. Returns a boolean.
`$range`	Outputs an array containing a sequence of integers according to user-defined inputs.
`$reverseArray`	Returns an array with the elements in reverse order.
`$reduce`	Applies an expression to each element in an array and combines them into a single value.
`$size`	Returns the number of elements in the array. Accepts a single expression as argument.
`$slice`	Returns a subset of an array.
`$zip`	Merge two lists together.
`$in`	Returns a boolean indicating whether a specified value is in an array.

Variable Expressions¶

Name	描述
`$map`	Applies a subexpression to each element of an array and returns the array of resulting values in order. Accepts named parameters.
`$let`	Defines variables for use within the scope of a subexpression and returns the result of the subexpression. Accepts named parameters.

Literal Expressions¶

Name	描述
`$literal`	Return a value without parsing. Use for values that the aggregation pipeline may interpret as an expression. For example, use a `$literal` expression to a string that starts with a `$` to avoid parsing as a field path.

Data Type Expressions¶

Name	描述
`$type`	Return the BSON data type of the field.

Date Expressions¶

Name	描述
`$dayOfYear`	Returns the day of the year for a date as a number between 1 and 366 (leap year).
`$dayOfMonth`	Returns the day of the month for a date as a number between 1 and 31.
`$dayOfWeek`	Returns the day of the week for a date as a number between 1 (Sunday) and 7 (Saturday).
`$year`	Returns the year for a date as a number (e.g. 2014).
`$month`	Returns the month for a date as a number between 1 (January) and 12 (December).
`$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).
`$hour`	Returns the hour for a date as a number between 0 and 23.
`$minute`	Returns the minute for a date as a number between 0 and 59.
`$second`	Returns the seconds for a date as a number between 0 and 60 (leap seconds).
`$millisecond`	Returns the milliseconds of a date as a number between 0 and 999.
`$dateToString`	Returns the date as a formatted string.
`$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).

Conditional Expressions¶

Name	描述
`$cond`	A ternary operator that evaluates one expression, and depending on the result, returns the value of one of the other two expressions. Accepts either three expressions in an ordered list or three named parameters.
`$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.

Accumulators¶

在 3.2 版更改: Some accumulators are now available in the $project stage. In previous versions of MongoDB , accumulators are available only for the $group stage.

Accumulators, when used in the $group stage, maintain their state (e.g. totals, maximums, minimums, and related data) as documents progress through the pipeline.

When used in the $group stage, accumulators take as input a single expression, evaluating the expression once for each input document, and maintain their stage for the group of documents that share the same group key.

When used in the $project stage, the accumulators do not maintain their state. When used in the $project stage, accumulators take as input either a single argument or multiple arguments.

Name	描述
`$sum`	Returns a sum of numerical values. Ignores non-numeric values. 在 3.2 版更改: Available in both `$group` and `$project` stages.
`$avg`	Returns an average of numerical values. Ignores non-numeric values. 在 3.2 版更改: Available in both `$group` and `$project` stages.
`$first`	Returns a value from the first document for each group. Order is only defined if the documents are in a defined order. Available in `$group` stage only.
`$last`	Returns a value from the last document for each group. Order is only defined if the documents are in a defined order. Available in `$group` stage only.
`$max`	Returns the highest expression value for each group. 在 3.2 版更改: Available in both `$group` and `$project` stages.
`$min`	Returns the lowest expression value for each group. 在 3.2 版更改: Available in both `$group` and `$project` stages.
`$push`	Returns an array of expression values for each group. Available in `$group` stage only.
`$addToSet`	Returns an array of unique expression values for each group. Order of the array elements is undefined. Available in `$group` stage only.
`$stdDevPop`	Returns the population standard deviation of the input values. 在 3.2 版更改: Available in both `$group` and `$project` stages.
`$stdDevSamp`	Returns the sample standard deviation of the input values. 在 3.2 版更改: Available in both `$group` and `$project` stages.

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