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# \$1sum
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Amazon DocumentDB 中的`$sum`运算符返回一组中每个文档的指定表达式的总和。它是一个组累加器运算符，通常用于聚合管道的 \$1group 阶段来执行求和计算。

**参数**
+ `expression`：要求和的数值表达式。这可以是字段路径、表达式或常量。

## 示例（MongoDB 外壳）
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以下示例演示如何使用运`$sum`算符来计算每种产品的总销售额。

**创建示例文档**

```
db.sales.insertMany([
  { product: "abc", price: 10, quantity: 2 },
  { product: "abc", price: 10, quantity: 3 },
  { product: "xyz", price: 20, quantity: 1 },
  { product: "xyz", price: 20, quantity: 5 }
]);
```

**查询示例**

```
db.sales.aggregate([
  { $group: {
      _id: "$product",
      totalSales: { $sum: { $multiply: [ "$price", "$quantity" ] } }
    }}
]);
```

**输出**

```
[
  { "_id": "abc", "totalSales": 50 },
  { "_id": "xyz", "totalSales": 120 }
]
```

## 代码示例
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要查看使用该`$sum`命令的代码示例，请选择要使用的语言的选项卡：

------
#### [ Node.js ]

```
const { MongoClient } = require('mongodb');

async function example() {
  const uri = 'mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false';
  const client = new MongoClient(uri);

  try {
    await client.connect();

    const db = client.db('test');
    const collection = db.collection('sales');

    const result = await collection.aggregate([
      { $group: {
          _id: "$product",
          totalSales: { $sum: { $multiply: [ "$price", "$quantity" ] } }
        }}
    ]).toArray();

    console.log(result);

  } catch (error) {
    console.error('Error:', error);
  } finally {
    await client.close();
  }
}

example();
```

------
#### [ Python ]

```
from pymongo import MongoClient
from pprint import pprint

def example():
    client = None
    try:
        client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')

        db = client.test
        collection = db.sales

        result = list(collection.aggregate([
            { '$group': {
                '_id': '$product',
                'totalSales': { '$sum': { '$multiply': [ '$price', '$quantity' ] } }
            }}
        ]))

        pprint(result)

    except Exception as e:
        print(f"An error occurred: {e}")

    finally:
        if client:
            client.close()

example()
```

------