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# \$1group
<a name="group"></a>

Amazon DocumentDB 中的`$group`聚合阶段允许您按指定的表达式对文档进行分组，并对分组的数据执行各种累积操作。这对于诸如根据分组数据计算总计、平均值或其他统计数据之类的任务非常有用。

**参数**
+ `_id`：指定对输入文档进行分组时应采用的表达式。这可以是字段名、计算表达式或两者的组合。
+ `accumulator expressions`:（可选）应应用于分组数据的一个或多个累加器表达式。这些表达式使用上面提到的累加器运算符。

## 示例（MongoDB 外壳）
<a name="group-examples"></a>

以下示例按城市对客户进行分组，并计算每个城市的总订单金额。

**创建示例文档**

```
db.customers.insertMany([
  { name: "John Doe", city: "New York", orders: [{ amount: 100 }, { amount: 200 }] },
  { name: "Jane Smith", city: "Los Angeles", orders: [{ amount: 150 }, { amount: 300 }] },
  { name: "Bob Johnson", city: "New York", orders: [{ amount: 75 }, { amount: 125 }] },
  { name: "Samantha Lee", city: "Chicago", orders: [{ amount: 50 }, { amount: 100 }] }
]);
```

**查询示例**

```
db.customers.aggregate([
  {
    $group: {
      _id: "$city",
      totalOrders: { $sum: { $sum: "$orders.amount" } }
    }
  }
]);
```

**输出**

```
[
  { _id: 'Chicago', totalOrders: 150 },
  { _id: 'Los Angeles', totalOrders: 450 },
  { _id: 'New York', totalOrders: 500 }
]
```

## 代码示例
<a name="group-code"></a>

要查看使用该`$group`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 groupByCity() {
  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 result = await db.collection('customers').aggregate([
      { $unwind: '$orders' },
      {
        $group: {
          _id: '$city',
          totalOrders: { $sum: '$orders.amount' }
        }
      }
    ]).toArray();

    console.log(result);
  } catch (err) {
    console.error('Error during aggregation:', err);
  } finally {
    await client.close();
  }
}

groupByCity();
```

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

```
from pymongo import MongoClient

def group_by_city():
    client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
    
    try:
        db = client.test
        result = list(db.customers.aggregate([
            {'$unwind': '$orders'},
            {
                '$group': {
                    '_id': '$city',
                    'totalOrders': {'$sum': '$orders.amount'}
                }
            }
        ]))
        print(result)
    except Exception as e:
        print(f"Error during aggregation: {e}")
    finally:
        client.close()

group_by_city()
```

------