$bottom (aggregation accumulator)

Null and Missing Values空值和缺失值

Consider the following aggregation that returns the bottom document from a group of scores:考虑以下从一组分数中返回底部文档的聚合：

• $bottom does not filter out null values.不筛选空值。
• $bottom converts missing values to null.将缺少的值转换为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:
            {
               $bottom:
                  {
                     output: [ "$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。
• $group groups the documents by gameId. 按gameId对文档进行分组。This example has only one gameId, G1.这个例子只有一个gameId，G1。
• PlayerD has a missing score and PlayerE has a null score. 缺少分数，PlayerE的score为空。These values are both considered as null.这些值都被视为空。
• The playerId and score fields are specified as output : ["$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 bottom element. PlayerD is returned as the bottom score.PlayerD和PlayerE为底部元素并列。返回PlayerD作为最低score。
• To have more deterministic tie breaking behavior for multiple null values, add more fields to``sortBy``.要对多个空值具有更确定的平局打破行为，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

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

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

Find the Bottom 找到最低Score

You can use the $bottom accumulator to find the bottom score in a single game.您可以使用$bottom累加器查找单个游戏中的最低分数。

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

The example pipeline:示例管道：

• Uses $match to filter the results on a single gameId. 使用$match筛选单个gameId的结果。In this case, G1.在这种情况下为G1。
• Uses $group to group the results by gameId. 使用$group按gameId对结果进行分组。In this case, G1.在这种情况下为G1。
• Specifies the fields that are output for $bottom with output : ["$playerId"," $score"].指定为$bottom输出的字段，output : ["$playerId"," $score"]。
• 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 $bottom to return the bottom score for the game.使用$bottom返回游戏的最低分数。

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

[ { _id: 'G1', playerId: [ 'PlayerD', 1 ] } ]

Finding the Bottom Score Across Multiple Games在多个游戏中找到最低Score

You can use the $bottom accumulator to find the bottom score in each game.您可以使用$bottom累加器查找每场游戏的最低Score。

db.gamescores.aggregate( [
      {
         $group:
         { _id: "$gameId", playerId:
            {
               $bottom:
                  {
                     output: [ "$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 $bottom to return the bottom score for each game.使用$bottom返回每个游戏的最低score。
• Specifies the fields that are output for $bottom with output : ["$playerId", "$score"].指定为$bottom输出的字段，output : ["$playerId", "$score"]。
• 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: [ 'PlayerA', 10 ] },
   { _id: 'G1', playerId: [ 'PlayerD', 1 ] }
]