$bottomN (accumulator operator)（累加器运算符）

Null and Missing Valuesnull值和缺失值

• $bottomN does not filter out null values.不筛选null值。
• $bottomN converts missing values to null which are preserved in the output.将缺失的值转换为null，并保留在输出中。

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:
               {
                  $bottomN:
                     {
                        output: [ "$playerId", "$score" ],
                        sortBy: { "score": -1 },
                        n: 3
                     }
               }
      }
   }
] )

In this example:在这个例子中：

• $documents creates the literal documents that contain player scores.创建包含玩家分数的文字文档。
• $group groups the documents by gameId. This example has only one gameId, G1.按gameId对文档进行分组。这个例子只有一个gameId G1。
• PlayerD has a missing score and PlayerE has a null score. These values are both considered as null.PlayerD缺少score，PlayerE的score为空。这些值都被视为null。
• The playerId and score fields are specified as output : ["$playerId"," $score"] and returned as array values.playerId和score字段被指定为output : ["$playerId"," $score"]，并作为数组值返回。
• Because of the sortBy: { "score" : -1 }, the null values are sorted to the end of the returned playerId array.由于sortBy: { "score" : -1 }，null值被排序到返回的playerId数组的末尾。

[
   {
      _id: "G1",
      playerId: [ [ "PlayerA", 1 ], [ "PlayerD", null ], [ "PlayerE", null ] ]
   }
]

BSON Data Type Sort OrderingBSON数据类型排序

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.在对不同类型进行排序时，BSON数据类型的顺序用于确定排序。例如，考虑一个值由字符串和数字组成的集合。

• In an ascending sort, string values are sorted after numeric values.在升序排序中，字符串值在数值之后排序。
• In a descending sort, string values are sorted before numeric values.在降序排序中，字符串值在数值之前排序。

db.aggregate( [
   {
      $documents: [
         { playerId: "PlayerA", gameId: "G1", score: 1 },
         { playerId: "PlayerB", gameId: "G1", score: "2" },
         { playerId: "PlayerC", gameId: "G1", score: "" }
      ]
   },
   {
      $group:
         {
            _id: "$gameId",
            playerId: {
               $bottomN:
               {
                  output: ["$playerId","$score"],
                  sortBy: {"score": -1},
                  n: 3
               }
            }
         }
   }
] )

In this example:在这个例子中：

• PlayerA has an integer score.具有整数分数。
• PlayerB has a string "2" score.具有字符串"2"分数。
• PlayerC has an empty string score.具有空字符串分数。

Because the sort is in descending { "score" : -1 }, the string literal values are sorted before PlayerA's numeric score:因为排序是降序{ "score" : -1 }，所以字符串文字值在PlayerA的数字分数之前排序：

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

Window Function and Aggregation Expression Support窗口函数和聚合表达式支持

$bottomN is not supported as a aggregation expression.$bottomN不支持作为聚合表达式。

$bottomN is supported as a window operator.$bottomN被支持作为窗口运算符。

Memory Limit Considerations内存限制注意事项

Groups within the $bottomN aggregation pipeline are subject to the 100 MB limit pipeline limit. If this limit is exceeded for an individual group, the aggregation fails with an error.$bottomN聚合管道内的组受100MB限制管道限制。如果单个组超过此限制，聚合将失败并出现错误。

Find the Three Lowest Scores找出三个最低的Scores

You can use the $bottomN accumulator to find the lowest scoring players in a single game.您可以使用$bottomN累加器来查找单场比赛中得分最低的球员。

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

The example pipeline:示例管道：

• Uses $match to filter the results on a single gameId. In this case, G1.使用$match筛选单个gameId的结果。在这种情况下，G1。
• Uses $group to group the results by gameId. In this case, G1.使用$group按gameId对结果进行分组。在这种情况下，G1。
• Uses sort by { "score": -1 } to sort the results in descending order.使用{ "score": -1 }排序，按降序对结果进行排序。
• Specifies the fields that are output from $bottomN with output : ["$playerId"," $score"].使用output : ["$playerId"," $score"]指定从$bottomN输出的字段。
• Uses $bottomN to return the bottom three documents with the lowest score for the G1 game with n : 3.使用$bottomN返回n为3的G1游戏score最低的底部三个文档。

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

[
   {
      _id: "G1",
      playerId: [ [ "PlayerB", 33 ], [ "PlayerA", 31 ], [ "PlayerD", 1 ] ]
   }
]

The SQL equivalent to this query is:与此查询等效的SQL是：

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) AS T3
            ON GS.GAMEID = T3.GAMEID
GROUP BY T3.GAMEID,T3.PLAYERID,T3.SCORE
   ORDER BY T3.SCORE DESC

Finding the Three Lowest Score Documents Across Multiple Games在多个游戏中找到三个得分最低的文件

You can use the $bottomN accumulator to find the lowest scoring players in each game.您可以使用$bottomN累加器来查找每场比赛中得分最低的球员。

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

The example pipeline:示例管道：

• Uses $group to group the results by gameId.使用$group按gameId对结果进行分组。
• Specifies the fields that are output from $bottomN with output : ["$playerId", "$score"].使用output : ["$playerId", "$score"]指定从$bottomN输出的字段。
• Uses sort by { "score": -1 } to sort the results in descending order.使用{ "score": -1 }排序，按降序对结果进行排序。
• Uses $bottomN to return the bottom three documents with the lowest score for each game with n: 3.使用$bottomN配合n:3返回每个游戏score最低的底部三个文档。

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

[
   {
      _id: "G1",
      playerId: [ [ "PlayerB", 33 ], [ "PlayerA", 31 ], [ "PlayerD", 1 ] ]
   },
   {
      _id: "G2",
      playerId: [ [ "PlayerC", 66 ], [ "PlayerB", 14 ], [ "PlayerA", 10 ] ]
   }
]

The SQL equivalent to this query is:与此查询等效的SQL是：

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 >= 2
ORDER BY GAMEID

Computing n Based on the Group Key for $group基于$Group的组键计算n

You can also assign the value of n dynamically. 您还可以动态地分配n的值。In this example, the $cond expression is used on the gameId field.在这个例子中，$cond表达式用于gameId字段。

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

The example pipeline:示例管道：

• Use $group to group the results by gameId.使用$group按gameId对结果进行分组。
• Specifies the fields that are output from $bottomN with output : "$score".使用output : "$score"指定从$bottomN输出的字段。
• If the gameId is G2 then n is 1, otherwise n is 3.如果gameId为G2，则n为1，否则n为3。
• Uses sort by { "score": -1 } to sort the results in descending order.使用{ "score": -1 }排序，按降序对结果进行排序。

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

[
   { _id: { gameId: "G2" }, gamescores: [ 10 ] },
   { _id: { gameId: "G1" }, gamescores: [ 33, 31, 1 ] }
]