$sum (accumulator operator)（累加器运算符）

Result Data Type结果数据类型

When input types are mixed, $sum promotes the smaller input type to the larger of the two. A type is considered larger when it represents a wider range of values. The order of numeric types from smallest to largest is: 当输入类型混合时，$sum会将较小的输入类型升级为两者中较大的输入类型。当一个类型代表更宽的值范围时，它被认为更大。数字类型从最小到最大的顺序是：integer → long → double → decimal

The larger of the input types also determines the result type unless the operation overflows and is beyond the range represented by that larger data type. In cases of overflow, $sum promotes the result according to the following order:较大的输入类型也决定了结果类型，除非操作溢出并且超出了较大数据类型表示的范围。在溢出的情况下，$sum会按照以下顺序提升结果：

• If the larger input type is integer, the result type is promoted to long.如果较大的输入类型是integer，则结果类型将提升为long。
• If the larger input type is long, the result type is promoted to double.如果较大的输入类型为long，则结果类型将提升为double。
• If the larger type is double or decimal, the overflow result is represented as + or - infinity. There is no type promotion of the result.如果较大的类型是double或decimal，则溢出结果表示为+或-无穷大。结果没有类型推广。

Non-Numeric or Non-Existent Fields非数字或不存在字段

If used on a field that contains both numeric and non-numeric values, $sum ignores the non-numeric values and returns the sum of the numeric values.如果在同时包含数值和非数值的字段上使用，$sum会忽略非数值，并返回数值的总和。

If used on a field that does not exist in any document in the collection, $sum returns 0 for that field.如果用于集合中任何文档中都不存在的字段，$sum将为该字段返回0。

If all operands are non-numeric, non-arrays, or contain null values, $sum returns 0. For details on how $sum handles arrays, see Array Operand.如果所有操作数都是非数字、非数组或包含null值，则$sum返回0。有关$sum如何处理数组的详细信息，请参阅数组运算数。

Array Operand数组运算数

In the $group stage, if the expression resolves to an array, $sum treats the operand as a non-numeric value.在$group阶段，如果表达式解析为数组，$sum会将操作数视为非数值。

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, $sum traverses into the array to operate on the numeric elements of the array to return a single value.使用单个表达式作为操作数，如果表达式解析为数组，$sum将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, $sum does not traverse into the array but instead treats the array as a non-numeric value.使用表达式列表作为操作数，如果任何表达式解析为数组，$sum不会遍历到数组中，而是将数组视为非数值。

For example, when not used in a $group stage:例如，当不在$group阶段使用时：

• If the $sum operand is [ 2, 2 ], $sum adds the array elements and returns 4.如果$sum操作数为[2,2]，则$sum将数组元素相加并返回4。
• If the $sum operand is [ 2, [ 3, 4 ] ], $sum returns 2 because it treats the nested array [ 3, 4 ] as a non-numeric value.如果$sum操作数为[2,[3,4]]，则$sum返回2，因为它将嵌套数组[3,4]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销售集合：

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 day and the year of the date field, the following operation uses the $sum accumulator to compute the total amount and the count for each group of documents.按date字段的日期和年份对文档进行分组，以下操作使用$sum累加器计算每组文档的总金额和计数。

db.sales.aggregate(
   [
     {
       $group:
         {
           _id: { day: { $dayOfYear: "$date"}, year: { $year: "$date" } },
           totalAmount: { $sum: { $multiply: [ "$price", "$quantity" ] } },
           count: { $sum: 1 }
         }
     }
   ]
)

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

{ "_id" : { "day" : 46, "year" : 2014 }, "totalAmount" : 150, "count" : 2 }
{ "_id" : { "day" : 34, "year" : 2014 }, "totalAmount" : 45, "count" : 2 }
{ "_id" : { "day" : 1, "year" : 2014 }, "totalAmount" : 20, "count" : 1 }

Using $sum on a non-existent field returns a value of 0. The following operation attempts to $sum on qty:对不存在的字段使用$sum将返回值0。以下操作尝试按qty计算$sum：

db.sales.aggregate(
   [
     {
       $group:
         {
           _id: { day: { $dayOfYear: "$date"}, year: { $year: "$date" } },
           totalAmount: { $sum: "$qty" },
           count: { $sum: 1 }
         }
     }
   ]
)

The operation returns:操作返回：

{ "_id" : { "day" : 46, "year" : 2014 }, "totalAmount" : 0, "count" : 2 }
{ "_id" : { "day" : 34, "year" : 2014 }, "totalAmount" : 0, "count" : 2 }
{ "_id" : { "day" : 1, "year" : 2014 }, "totalAmount" : 0, "count" : 1 }

The $count aggregation accumulator can be used in place of { $sum : 1 } in the $group stage.$count聚合累加器可以用来代替$group阶段中的{ $sum : 1 }。

Use in $project Stage在$project阶段中使用

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

{ "_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 $sum in the $project stage to calculate the total quiz scores, the total lab scores, and the total of the final and the midterm:以下示例使用$project阶段中的$sum来计算测验总分、实验总分以及期末和期中考试的总分：

db.students.aggregate([
   {
     $project: {
       quizTotal: { $sum: "$quizzes"},
       labTotal: { $sum: "$labs" },
       examTotal: { $sum: [ "$final", "$midterm" ] }
     }
   }
])

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

{ "_id" : 1, "quizTotal" : 23, "labTotal" : 13, "examTotal" : 155 }
{ "_id" : 2, "quizTotal" : 19, "labTotal" : 16, "examTotal" : 175 }
{ "_id" : 3, "quizTotal" : 14, "labTotal" : 11, "examTotal" : 148 }

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 $sum in the $setWindowFields stage to output the sum of the quantity of cakes sold in each state:此示例在$setWindowFields阶段使用$sum来输出每个state销售的蛋糕quantity的总和：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            sumQuantityForState: {
               $sum: "$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 sumQuantityForState field to the sum of the quantity values using $sum that is run in a documents window.使用在documents窗口中运行的$sum将sumQuantityForState字段设置为quantity值的总和。

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

In this output, the sum of the quantity values for CA and WA is shown in the sumQuantityForState field:在此输出中，CA和WA的quantity值之和显示在sumQuantityForState字段中：

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "sumQuantityForState" : 162 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "sumQuantityForState" : 282 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "sumQuantityForState" : 427 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "sumQuantityForState" : 134 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "sumQuantityForState" : 238 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "sumQuantityForState" : 378 }