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$maxN（表达式操作符）

定义

$maxN: 5.2版本新增。
返回数组中 n 个最大值。

提示

$minN

语法

$maxN 通过以下语法实现：

{ $maxN: { n: <expression>, input: <expression> } }

字段	说明
`n`	An expression that resolves to a positive integer. The integer specifies the number of array elements that `$maxN` returns.
`input`	一个表达式，可解析为要返回最多`n`元素的大量。

行为

不能指定小于 1 的 n 值。
$maxN filters out null values found in the input array.
If the specified n is greater than or equal to the number of elements in the input array, $maxN returns all elements in the input array.
如果 input 解析为非数组值，则聚合操作将出错。
如果input 同时包含数字和string 元素，则根据string BSON比较顺序，元素排在数字元素之前。

例子

使用以下文档创建 scores 集合：

db.scores.insertMany([
    { "playerId" : 1, "score" : [ 1, 2, 3 ] },
    { "playerId" : 2, "score" : [ 12, 90, 7, 89, 8 ] },
    { "playerId" : 3, "score" : [ null ] },
    { "playerId" : 4, "score" : [ ] }
    { "playerId" : 5, "score" : [ 1293, "2", 3489, 9 ]}
])

The following example uses the $maxN operator to retrieve the two highest scores for each player. The highest scores are returned in the new field maxScores created by $addFields.

db.scores.aggregate([
   { $addFields: { maxScores: { $maxN: { n: 2, input: "$score" } } } }
])

操作返回以下结果：

[{
  "playerId": 1,
  "score": [ 1, 2, 3 ],
  "maxScores": [ 3, 2 ]
},
{
  "playerId": 2,
  "score": [ 12, 90, 7, 89, 8 ],
  "maxScores": [ 90, 89 ]
},
{
  "playerId": 3,
  "score": [ null ],
  "maxScores": [ ]
},
{
  "playerId": 4,
  "score": [ ],
  "maxScores": [ ]
},
{
  "playerId": 5,
  "score": [ 1293, "2", 3489, 9 ],
  "maxScores": [ "2", 3489 ]
}]