$split (aggregation)

Definition定义

$split

Divides a string into an array of substrings based on a delimiter. 基于分隔符将字符串划分为子字符串数组。$split removes the delimiter and returns the resulting substrings as elements of an array. 删除分隔符并将结果子字符串作为数组的元素返回。If the delimiter is not found in the string, $split returns the original string as the only element of an array.如果在字符串中找不到分隔符，$split将返回原始字符串作为数组的唯一元素。

$split has the following operator expression syntax:具有以下运算符表达式语法：

{ $split: [ <string expression>, <delimiter> ] }

Field字段	Type类型	Description描述
string expression	string	The string to be split. 要拆分的字符串。string expression can be any valid expression as long as it resolves to a string. 可以是任何有效的表达式，只要它解析为字符串。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。
delimiter	string	The delimiter to use when splitting the string expression. 拆分字符串表达式时要使用的分隔符。delimiter can be any valid expression as long as it resolves to a string.可以是任何有效的表达式，只要它解析为字符串。

Behavior行为

The $split operator returns an array. $split运算符返回一个数组。The <string expression> and <delimiter> inputs must both be strings. <string expression>和<delimiter>输入都必须是字符串。Otherwise, the operation fails with an error.否则，操作将失败并返回错误。

{ $split: [ "June-15-2013", "-" ] }

[ "June", "15", "2013" ]

{ $split: [ "banana split", "a" ] }

[ "b", "n", "n", " split" ]

{ $split: [ "Hello World", " " ] }

[ "Hello", "World" ]

{ $split: [ "astronomical", "astro" ] }

[ "", "nomical" ]

{ $split: [ "pea green boat", "owl" ] }

[ "pea green boat" ]

Example示例

A collection named deliveries contains the following documents:名为deliveries的集合包含以下文档：

{ "_id" : 1, "city" : "Berkeley, CA", "qty" : 648 }
{ "_id" : 2, "city" : "Bend, OR", "qty" : 491 }
{ "_id" : 3, "city" : "Kensington, CA", "qty" : 233 }
{ "_id" : 4, "city" : "Eugene, OR", "qty" : 842 }
{ "_id" : 5, "city" : "Reno, NV", "qty" : 655 }
{ "_id" : 6, "city" : "Portland, OR", "qty" : 408 }
{ "_id" : 7, "city" : "Sacramento, CA", "qty" : 574 }

The goal of following aggregation operation is to find the total quantity of deliveries for each state and sort the list in descending order. 以下聚合操作的目标是查找每个州的交付总量，并按降序对列表进行排序。It has five pipeline stages:它有五个管道阶段：

• The $project stage produces documents with two fields, qty (integer) and city_state (array). $project阶段生成具有两个字段的文档，qty（integer）和city_state（array）。The $split operator creates an array of strings by splitting the city field, using a space (" ") as a delimiter.$split运算符使用空格（" "）作为分隔符，通过拆分城市字段来创建字符串数组。
• The $unwind stage creates a separate record for each element in the city_state field.$unwind阶段为city_state字段中的每个元素创建单独的记录。
• The $match stage uses a regular expression to filter out the city documents, leaving only those containing a state.$match阶段使用正则表达式筛选掉城市文档，只留下那些包含州的文档。
• The $group stage groups all the states together and sums the qty field.$group阶段将所有州组合在一起，并对qty字段求和。
• The $sort stage sorts the results by total_qty in descending order.$sort阶段按total_qty降序对结果进行排序。

db.deliveries.aggregate([
  { $project : { city_state : { $split: ["$city", ", "] }, qty : 1 } },
  { $unwind : "$city_state" },
  { $match : { city_state : /[A-Z]{2}/ } },
  { $group : { _id: { "state" : "$city_state" }, total_qty : { "$sum" : "$qty" } } },
  { $sort : { total_qty : -1 } }
]);

The operation returns the following results:该操作返回以下结果：

{ "_id" : { "state" : "OR" }, "total_qty" : 1741 }
{ "_id" : { "state" : "CA" }, "total_qty" : 1455 }
{ "_id" : { "state" : "NV" }, "total_qty" : 655 }