$sum (aggregation)

Result Data Type结果数据类型

The result will have the same type as the input except when it cannot be represented accurately in that type. In these cases:结果将具有与输入相同的类型，除非无法在该类型中准确表示。在这些情况下：

• A 32-bit integer will be converted to a 64-bit integer if the result is representable as a 64-bit integer.如果结果可以表示为64位整数，则32位整数将被转换为64位的整数。
• A 32-bit integer will be converted to a double if the result is not representable as a 64-bit integer.如果结果不能表示为64位整数，则32位整数将转换为双精度。
• A 64-bit integer will be converted to double if the result is not representable as a 64-bit integer.如果结果不能表示为64位整数，则64位整数将被转换为双。

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

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将忽略非数值并返回数值的和。

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

If all operands are non-numeric, $sum returns 0.如果所有操作数都是非数字的，那么$sum将返回0。

Array Operand数组操作数

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

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档的sales集合：

{ "_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 $sum accumulator to compute the total amount and the count for each group of documents.根据date字段的日期和年份对文档进行分组，以下操作使用$sum累加器来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 $sum on a non-existent field returns a value of 0. 对不存在的字段使用$sum将返回值0。The following operation attempts to $sum on qty:以下操作尝试对qty进行$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阶段的$sum来计算测验总分、实验室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:操作产生以下文件：

{ "_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):创建一个包含加利福尼亚州（CA）和华盛顿州（WA）蛋糕销售的cakeSales系列：

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 $setWindowFields stage to output the sum of the quantity of cakes sold in each state:此示例使用$setWindowFields阶段中的$sum来输出每个state下售出的蛋糕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. 按state对集合中的文档进行分区。There are partitions for CA and WA.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按升序（1）对每个分区中的文档进行排序，因此最早的orderDate是第一个。

• output sets the sumQuantityForState field to the sum of the quantity values using $sum that is run in a documents window.output使用在文档窗口中运行的$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. 该窗口包含在unbounded下限和输出中的current文档之间的文档。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:在该输出中，CA和WA的数量值之和显示在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 }