$max (accumulator operator)（累加器运算符）

Result Type结果类型

$max compares input data following the BSON Comparison order to determine the appropriate output type. $max按照BSON比较顺序比较输入数据，以确定适当的输出类型。When the input data contains multiple data types, the $max output type is the highest in the comparison order.当输入数据包含多种数据类型时，$max输出类型在比较顺序中最高。

Null or Missing Values值为空或缺失

If some, but not all, documents for the $max operation have either a null value for the field or are missing the field, the $max operator only considers the non-null and the non-missing values for the field.如果$max操作的某些文档（但不是全部）的字段值为空或缺少字段，则$max运算符只考虑字段的非null值和非缺失值。

If all documents for the $max operation have null value for the field or are missing the field, the $max operator returns null for the maximum value.如果$max操作的所有文档的字段值为空或缺少字段，则$max运算符将返回最大值为空。

Array Operand数组操作对象

In the $group and $setWindowFields stages, if the expression resolves to an array, $max does not traverse the array and compares the array as a whole.在$group和$setWindowFields阶段，如果表达式解析为数组，则$max不会遍历数组，而是将数组作为一个整体进行比较。

In the other supported stages:在其他支持的阶段：

• With a single expression as its operand, if the expression resolves to an array, $max traverses into the array to operate on the numeric elements of the array to return a single value.以单个表达式作为操作数，如果表达式解析为数组，则$max遍历数组以对数组的数字元素进行操作，从而返回单个值。
• With a list of expressions as its operand, if any of the expressions resolves to an array, $max does not traverse into the array but instead treats the array as a non-numeric value.以表达式列表作为操作数，如果任何表达式解析为数组，则$max不会遍历到数组中，而是将数组视为非数值。

Use in $group Stage在$group阶段中使用

Consider a sales collection with the following documents:考虑一个包含以下文件的sales集合：

db.sales.insertMany ( [
   { _id : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") },
   { _id : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") },
   { _id : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") },
   { _id : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") },
   { _id : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:05:00Z") }
] )

Grouping the documents by the item field, the following operation uses the $max accumulator to compute the maximum total amount and maximum quantity for each group of documents.按item字段对文档进行分组，以下操作使用$max累加器计算每组文档的最大总金额和最大数量。

db.sales.aggregate(
   [
     {
       $group:
         {
           _id: "$item",
           maxTotalAmount: { $max: { $multiply: [ "$price", "$quantity" ] } },
           maxQuantity: { $max: "$quantity" }
         }
     }
   ]
)

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

{ "_id" : "xyz", "maxTotalAmount" : 50, "maxQuantity" : 10 }
{ "_id" : "jkl", "maxTotalAmount" : 20, "maxQuantity" : 1 }
{ "_id" : "abc", "maxTotalAmount" : 100, "maxQuantity" : 10 }

Use in $project Stage在$project阶段使用

A collection students contains the following documents:students集合包含以下文件：

db.students.insertMany( [
   { _id: 1, "quizzes": [ 10, 6, 7 ], "labs": [ 5, 8 ], "final": 80, "midterm": 75 },
   { _id: 2, "quizzes": [ 9, 10 ], "labs": [ 8, 8 ], "final": 95, "midterm": 80 },
   { _id: 3, "quizzes": [ 4, 5, 5 ], "labs": [ 6, 5 ], "final": 78, "midterm": 70 }
] )

The following example uses the $max in the $project stage to calculate the maximum quiz scores, the maximum lab scores, and the maximum of the final and the midterm:以下示例使用$project阶段中的$max来计算最大测验分数、最大实验室分数以及期末和期中考试的最大分数：

db.students.aggregate([
   { $project: { quizMax: { $max: "$quizzes"}, labMax: { $max: "$labs" }, examMax: { $max: [ "$final", "$midterm" ] } } }
])

The operation results in the following documents:该操作产生了以下文件：

{ "_id" : 1, "quizMax" : 10, "labMax" : 8, "examMax" : 80 }
{ "_id" : 2, "quizMax" : 10, "labMax" : 8, "examMax" : 95 }
{ "_id" : 3, "quizMax" : 5, "labMax" : 6, "examMax" : 78 }

Use in $setWindowFields Stage在$setWindowFields阶段中使用

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):创建一个cakeSales集合，其中包含加利福尼亚州（CA）和华盛顿州（WA）的蛋糕销售：

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

This example uses $max in the $setWindowFields stage to output the maximum quantity of cake sales for each state:此示例在$setWindowFields阶段使用$max来输出每个state的最大蛋糕quantity量：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            maximumQuantityForState: {
               $max: "$quantity",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   }
] )

In the example:在示例中：

• partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.按state对集合中的文档进行分区。CA和WA有分区。
• sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.按orderDate升序（1）对每个分区中的文档进行排序，因此最早的orderDate位居前列。

• output sets the maximumQuantityForState field to the maximum quantity value using $max that is run in a documents window.使用在documents窗口中运行的$max将maximumQuantityForState字段设置为最大quantity值。

  The window contains documents between an unbounded lower limit and the current document in the output. This means $max returns the maximum quantity for the documents between the beginning of the partition and the current document.该窗口包含的文档位于unbounded下限和输出中的current文档之间。这意味着$max返回分区开始和当前文档之间的文档的最大数量。

In this output, the maximum quantity for CA and WA is shown in the maximumQuantityForState field:在此输出中，CA和WA的最大quantity显示在maximumQuantityForState字段中：

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "maximumQuantityForState" : 162 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "maximumQuantityForState" : 162 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "maximumQuantityForState" : 162 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "maximumQuantityForState" : 134 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "maximumQuantityForState" : 134 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "maximumQuantityForState" : 140 }