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$split (aggregation)

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$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:

{ $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.

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"

A collection named deliveries contains the following documents:

db.deliveries.insertMany( [
{ _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 comma followed by 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.

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:

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

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