$top (aggregation accumulator)（聚合累加器）

Null and Missing Valuesnull值和缺失值

Consider the following aggregation that returns the top document from a group of scores:考虑以下聚合，它从一组分数中返回顶部文档：

• $top does not filter out null values.不筛选null值。
• $top converts missing values to null.将缺失的值转换为null。

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:
            {
               $top:
                  {
                     output: [ "$playerId", "$score" ],
                     sortBy: { "score": 1 }
                  }
            }
      }
   }
] )

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。
• $group groups the documents by gameId. This example has only one gameId, G1.按gameId对文档进行分组。这个例子只有一个gameId即G1。
• PlayerD has a missing score and PlayerE has a null score. These values are both considered as null.缺少score，而PlayerE的score为空。这些值都被视为null。
• The playerId and score fields are specified as output : ["$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。
• Specify the sort order with sortBy: { "score": 1 }.使用sortBy: { "score": 1 }指定排序顺序。
• PlayerD and PlayerE tied for the top element. PlayerD和PlayerE并列第一。PlayerD is returned as the top score.PlayerD作为最高score返回。
• To have more deterministic tie breaking behavior for multiple null values, add more fields to sortBy.为了对多个null值具有更确定的断开连接行为，请向sortBy添加更多字段。

[
   {
      _id: 'G1',
      playerId: [ 'PlayerD', null ]
   }
]

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

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

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

Memory Limit Considerations内存限制注意事项

Aggregation pipelines which call $top are subject to the 100 MB limit. If this limit is exceeded for an individual group, the aggregation fails with an error.调用$top的聚合管道受100MB限制。如果单个组超过此限制，聚合将失败并出现错误。

Find the Top Score找到最高Score

You can use the $top accumulator to find the top score in a single game.您可以使用$top累加器来查找单场比赛中的最高得分。

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

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。
• Specifies the fields that are output for $top with output : ["$playerId"," $score"].使用output : ["$playerId"," $score"]为$top指定输出字段。
• Uses sortBy: { "score": -1 } to sort the scores in descending order.使用sortBy: { "score": -1 }按降序对分数进行排序。
• Uses $top to return the top score in the game.使用$top返回游戏中的最高得分。

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

[ { _id: 'G1', playerId: [ 'PlayerC', 99 ] } ]

Find the Top Score Across Multiple Games在多个游戏中找到最高Score

You can use the $top accumulator to find the top score in each game.您可以使用$top累加器来计算每场比赛的最高score。

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

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。
• Uses $top to return top score for each game.使用$top返回每场比赛的最高score。
• Specifies the fields that are output for $top with output : ["$playerId", "$score"].使用output : ["$playerId", "$score"]为$top指定输出字段。
• Uses sortBy: { "score": -1 } to sort the scores in descending order.使用sortBy: { "score": -1 }按降序对分数进行排序。

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

[
   { _id: 'G2', playerId: [ 'PlayerD', 80 ] },
   { _id: 'G1', playerId: [ 'PlayerC', 99 ] }
]