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

Amazon DocumentDB 中的`$literal`運算子用於表示彙總管道階段中的常值。它可讓您包含特定值，例如數字、字串或布林值，而無需將其解譯為欄位參考或表達式。

當您需要在更複雜的彙總管道中包含常值時，例如建置動態查詢篩選條件或執行計算時，此運算子特別有用。

**參數**

無

## 範例 (MongoDB Shell)
<a name="literal-examples"></a>

下列範例示範如何使用 `$literal`運算子在彙總管道中包含常值。`$literal` 運算子用於在 \$1gt 表達式中包含值 18 作為常值。這可讓彙總管道將年齡欄位與常值 18 進行比較，以判斷該人員是否為成人。

**建立範例文件**

```
db.collection.insertMany([
  { "name": "John Doe", "age": 30, "city": "New York" },
  { "name": "Jane Doe", "age": 25, "city": "Los Angeles" },
  { "name": "Bob Smith", "age": 35, "city": "Chicago" }
]);
```

**查詢範例**

```
db.collection.aggregate([
  {
    $project: {
      name: 1,
      age: 1,
      city: 1,
      isAdult: { $gt: ["$age", { $literal: 18 }] }
    }
  }
]);
```

**輸出**

```
[
  {
    "_id": ObjectId("601234567890abcdef012345"),
    "name": "John Doe",
    "age": 30,
    "city": "New York",
    "isAdult": true
  },
  {
    "_id": ObjectId("601234567890abcdef012346"),
    "name": "Jane Doe",
    "age": 25,
    "city": "Los Angeles",
    "isAdult": true
  },
  {
    "_id": ObjectId("601234567890abcdef012347"),
    "name": "Bob Smith",
    "age": 35,
    "city": "Chicago",
    "isAdult": true
  }
]
```

## 程式碼範例
<a name="literal-code"></a>

若要檢視使用 `$literal`命令的程式碼範例，請選擇您要使用的語言標籤：

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

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

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

  const result = await collection.aggregate([
    {
      $project: {
        name: 1,
        age: 1,
        city: 1,
        isAdult: { $gt: ["$age", { $literal: 18 }] }
      }
    }
  ]).toArray();

  console.log(result);
  client.close();
}

example();
```

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

```
from pymongo import MongoClient

def example():
    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.collection

    result = list(collection.aggregate([
        {
            '$project': {
                'name': 1,
                'age': 1,
                'city': 1,
                'isAdult': { '$gt': ["$age", { '$literal': 18 }] }
            }
        }
    ]))

    print(result)
    client.close()

example()
```

------