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$split (aggregation)

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New in version 3.4.

Definition

$split

Divides a string into an array of substrings based on a delimiter. $split removes the delimiter and returns the resulting substrings as elements of an array. If the delimiter is not found in the string, $split returns the original string as the only element of an array.

$split has the following operator expression syntax:

copy 
{ $split: [ <string expression>, <delimiter> ] }
Field Type Description
string expression string The string to be split. string expression can be any valid expression as long as it resolves to a string. For more information on expressions, see Expressions.
delimiter string The delimiter to use when splitting the string expression. delimiter can be any valid expression as long as it resolves to a string.

Behavior

The $split operator returns an array. The <string expression> and <delimiter> inputs must both be strings. Otherwise, the operation fails with an error.

Example Results 
{ $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" ]
{ $split: [ "headphone jack", 7 ] }

Errors with message:

"$split requires an expression that evaluates to a string as a second argument, found: double"
{ $split: [ "headphone jack", /jack/ ] }

Errors with message:

"$split requires an expression that evaluates to a string as a second argument, found: regex"

Example

A collection named deliveries contains the following documents:

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{ "_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. It has five pipeline stages:

  • The $project stage produces documents with two fields, qty (integer) and city_state (array). The $split operator creates an array of strings by splitting the city field, using a space (" ") as a delimiter.
  • The $unwind stage creates a separate record for each element in the city_state field.
  • The $match stage uses a regular expression to filter out the city documents, leaving only those containing a state.
  • The $group stage groups all the states together and sums the qty field.
  • The $sort stage sorts the results by total_qty in descending order.
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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:

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

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