MongoDB聚合运算符：$bottomN

$bottomN聚合运算符返回分组中指定顺序的最后n个元素，如果分组中的元素数量小于n，则返回分组的全部元素。从MongoDB5.2开始支持。

语法

{
   $bottomN:
      {
         n: <expression>,
         sortBy: { <field1>: <sort order>, <field2>: <sort order> ... },
         output: <expression>
      }
}
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• n用于限制每组结果的数量，必须是正整数表达式，要么是常数，要么取决于$group的_id 值
• sortBy制定返回结果的顺序，语法类似于$sort
• output指定分组元素输出的内容，可以是任何合法的表达式。

用法

• $bottom不支持作为聚合表达式。
• $bottom只支持作为window 操作符。
• 聚合管道调用$bottom受100M的限制，如果单组超过这一限制将报错。

关于null和缺失值的处理

• $bottom不会过滤掉空值
• $bottom会将缺失值转换为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:
               {
                  $bottomN:
                     {
                        output: [ "$playerId", "$score" ],
                        sortBy: { "score": -1 },
                        n: 3
                     }
               }
      }
   }
] )
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在这个例子中：

• 使用$documents阶段创建了一些字面量（常量）文档，包含了选手的得分
• $group阶段根据gameId对文档进行了分组，显然文档中的gameId都是G1
• PlayerD的得分缺失，PlayerE的得分为null，他们的得分都会被当做null处理
• playerId字段和score字段被指定为输出：["$playerId"," $score"]，以数组的形式返回
• sortBy: { "score": -1 }指定了排序的方式，空值被排在最后，返回playerId数组

如下：

[
   {
      _id: "G1",
      playerId: [ [ "PlayerA", 1 ], [ "PlayerD", null ], [ "PlayerE", null ] ]
   }
]
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BSON数据类型排序

当不同类型排序是，使用BSON数据类型的顺序进行排序：

• 当进行正序排序时（由小到大），字符串的优先级在数值之前
• 当进行逆序排序时（由大到小），字符串的优先级在数值之前

下面的例子中包含了字符串和数值类型：

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
               }
            }
         }
   }
] )
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在这个例子中：

• PlayerA的得分是整数1
• PlayerB的得分是字符串"2"
• PlayerC的得分是空字符串""

因为排序指定为逆序{ "score" : -1 }，字符串的字面量排在PlayerA的数值得分之前：

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

使用下面的命令创建gamescores集合：

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 }
])
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查找三个得分最低的

使用$bottomN查找单个游戏中得分最低的3个：

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

• 使用$match阶段用一个gameId对结果进行筛选，即：G1
• 使用$group阶段依据gameId对结果进行分组，本例中只有一个分组G1
• 使用output : ["$playerId"," $score"]为bottom指定输出字段
• 使用sortBy: { "score": -1 }按照得分进行逆序排序
• 使用$bottomN返回游戏得分最低的3个选手和得分

结果如下：

[
   {
      _id: "G1",
      playerId: [ [ "PlayerB", 33 ], [ "PlayerA", 31 ], [ "PlayerD", 1 ] ]
   }
]
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与下面的SQL查询等价：

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
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查找全部游戏中三个最低的得分

使用$bottomN查找所有游戏中得分最低的三个

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

• 使用$group按照groupId对结果排序
• 使用output : ["$playerId", "$score"]指定bottom输出的字段
• 使用sortBy: { "score": -1 }按照得分进行逆序排序
• 使用$bottomN返回所有游戏中得分最低的三个

结果如下：

[
   {
      _id: "G1",
      playerId: [ [ "PlayerB", 33 ], [ "PlayerA", 31 ], [ "PlayerD", 1 ] ]
   },
   {
      _id: "G2",
      playerId: [ [ "PlayerC", 66 ], [ "PlayerB", 14 ], [ "PlayerA", 10 ] ]
   }
]
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这个操作与下面的SQL语句等价：

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
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基于分组key来计算参数n

可以动态指定n的值，在本例中$cond表达式用在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 }
                  }
            }
      }
   }
] )
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在本例中：

• 使用$group按照groupId对结果排序
• 使用output : "$score"指定$bottomN输出的字段
• 如果gameId是G2则n为1，否则n为3
• 使用sortBy: { "score": -1 }按照得分进行逆序排序

操作结果如下：

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