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$topN (aggregation accumulator)

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  • Definition
  • Syntax
  • Behavior
  • Restrictions
  • Examples
$topN

New in version 5.2.

Returns an aggregation of the top n elements within a group, according to the specified sort order. If the group contains fewer than n elements, $topN returns all elements in the group.

{
$topN:
{
n: <expression>,
sortBy: { <field1>: <sort order>, <field2>: <sort order> ... },
output: <expression>
}
}
  • n limits the number of results per group and has to be a positive integral expression that is either a constant or depends on the _id value for $group.
  • sortBy specifies the order of results, with syntax similar to $sort.
  • output represents the output for each element in the group and can be any expression.
  • $topN does not filter out null values.
  • $topN converts missing values to null which are preserved in the output.
db.aggregate( [
{
$documents: [
{ playerId: "PlayerA", gameId: "G1", score: 1 },
{ playerId: "PlayerB", gameId: "G1", score: 2 },
{ playerId: "PlayerC", gameId: "G1", score: 3 },
{ playerId: "PlayerD", gameId: "G1"},
{ playerId: "PlayerE", gameId: "G1", score: null }
]
},
{
$group:
{
_id: "$gameId",
playerId:
{
$topN:
{
output: [ "$playerId", "$score" ],
sortBy: { "score": 1 },
n: 3
}
}
}
}
] )

In this example:

  • $documents creates the literal documents that contain player scores.
  • $group groups the documents by gameId. This example has only one gameId, G1.
  • PlayerD has a missing score and PlayerE has a null score. These values are both considered as null.
  • The playerId and score fields are specified as output : ["$playerId"," $score"] and returned as array values.
  • Because of the sortBy: { "score" : 1 }, the null values are sorted to the front of the returned playerId array.
[
{
_id: 'G1',
playerId: [ [ 'PlayerD', null ], [ 'PlayerE', null ], [ 'PlayerA', 1 ] ]
}
]

When sorting different types, the order of BSON data types is used to determine ordering. As an example, consider a collection whose values consist of strings and numbers.

  • In an ascending sort, string values are sorted after numeric values.
  • In a descending sort, string values are sorted before numeric values.
db.aggregate( [
{
$documents: [
{ playerId: "PlayerA", gameId: "G1", score: 1 },
{ playerId: "PlayerB", gameId: "G1", score: "2" },
{ playerId: "PlayerC", gameId: "G1", score: "" }
]
},
{
$group:
{
_id: "$gameId",
playerId: {
$topN:
{
output: ["$playerId","$score"],
sortBy: {"score": -1},
n: 3
}
}
}
}
] )

In this example:

  • PlayerA has an integer score.
  • PlayerB has a string "2" score.
  • PlayerC has an empty string score.

Because the sort is in descending { "score" : -1 }, the string literal values are sorted before PlayerA's numeric score:

[
{
_id: "G1",
playerId: [ [ "PlayerB", "2" ], [ "PlayerC", "" ], [ "PlayerA", 1 ] ]
}
]

$topN is not supported as a aggregation expression.

$topN is supported as a window operator.

Groups within the $topN aggregation pipeline are subject to the 100 MB limit pipeline limit. If this limit is exceeded for an individual group, the aggregation fails with an error.

Consider a gamescores collection with the following documents:

db.gamescores.insertMany([
{ playerId: "PlayerA", gameId: "G1", score: 31 },
{ playerId: "PlayerB", gameId: "G1", score: 33 },
{ playerId: "PlayerC", gameId: "G1", score: 99 },
{ playerId: "PlayerD", gameId: "G1", score: 1 },
{ playerId: "PlayerA", gameId: "G2", score: 10 },
{ playerId: "PlayerB", gameId: "G2", score: 14 },
{ playerId: "PlayerC", gameId: "G2", score: 66 },
{ playerId: "PlayerD", gameId: "G2", score: 80 }
])

You can use the $topN accumulator to find the highest scoring players in a single game.

db.gamescores.aggregate( [
{
$match : { gameId : "G1" }
},
{
$group:
{
_id: "$gameId",
playerId:
{
$topN:
{
output: ["$playerId", "$score"],
sortBy: { "score": -1 },
n:3
}
}
}
}
] )

The example pipeline:

  • Uses $match to filter the results on a single gameId. In this case, G1.
  • Uses $group to group the results by gameId. In this case, G1.
  • Uses sort by { "score": -1 } to sort the results in descending order.
  • Specifies the fields that are output from $topN with output : ["$playerId"," $score"].
  • Uses $topN to return the top three documents with the highest score for the G1 game with n : 3.

The operation returns the following results:

[
{
_id: 'G1',
playerId: [ [ 'PlayerC', 99 ], [ 'PlayerB', 33 ], [ 'PlayerA', 31 ] ]
}
]

The SQL equivalent to this query is:

SELECT T3.GAMEID,T3.PLAYERID,T3.SCORE
FROM GAMESCORES AS GS
JOIN (SELECT TOP 3
GAMEID,PLAYERID,SCORE
FROM GAMESCORES
WHERE GAMEID = 'G1'
ORDER BY SCORE DESC) AS T3
ON GS.GAMEID = T3.GAMEID
GROUP BY T3.GAMEID,T3.PLAYERID,T3.SCORE
ORDER BY T3.SCORE DESC

You can use the $topN accumulator to find the highest scoring players in each game.

db.gamescores.aggregate( [
{
$group:
{ _id: "$gameId", playerId:
{
$topN:
{
output: [ "$playerId","$score" ],
sortBy: { "score": -1 },
n: 3
}
}
}
}
] )

The example pipeline:

  • Uses $group to group the results by gameId.
  • Specifies the fields that are output from $topN with output : ["$playerId", "$score"].
  • Uses sort by { "score": -1 } to sort the results in descending order.
  • Uses $topN to return the top three documents with the highest score for each game with n: 3.

The operation returns the following results:

[
{
_id: 'G1',
playerId: [ [ 'PlayerC', 99 ], [ 'PlayerB', 33 ], [ 'PlayerA', 31 ] ]
},
{
_id: 'G2',
playerId: [ [ 'PlayerD', 80 ], [ 'PlayerC', 66 ], [ 'PlayerB', 14 ] ]
}
]

The SQL equivalent to this query is:

SELECT PLAYERID,GAMEID,SCORE
FROM(
SELECT ROW_NUMBER() OVER (PARTITION BY GAMEID ORDER BY SCORE DESC) AS GAMERANK,
GAMEID,PLAYERID,SCORE
FROM GAMESCORES
) AS T
WHERE GAMERANK <= 3
ORDER BY GAMEID

You can also assign the value of n dynamically. In this example, the $cond expression is used on the gameId field.

db.gamescores.aggregate([
{
$group:
{
_id: {"gameId": "$gameId"},
gamescores:
{
$topN:
{
output: "$score",
n: { $cond: { if: {$eq: ["$gameId","G2"] }, then: 1, else: 3 } },
sortBy: { "score": -1 }
}
}
}
}
] )

The example pipeline:

  • Uses $group to group the results by gameId.
  • Specifies the fields that are output from $topN with output : "$score".
  • If the gameId is G2 then n is 1, otherwise n is 3.
  • Uses sort by { "score": -1 } to sort the results in descending order.

The operation returns the following results:

[
{ _id: { gameId: 'G1' }, gamescores: [ 99, 33, 31 ] },
{ _id: { gameId: 'G2' }, gamescores: [ 80 ] }
]
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