Documentdb Sql 简明教程

DocumentDB SQL - Value Keyword

当您知道您仅返回单个值时，VALUE 关键字可以通过避免创建完整对象开销来帮助生成精简的结果集。VALUE 关键字提供了一种返回 JSON 值的方法。

我们来看一个简单的例子。

value keyword

以下是包含 VALUE 关键字的查询。

SELECT VALUE "Hello World, this is DocumentDB SQL Tutorial"

当执行此查询时，他返回标量“Hello World，这是 DocumentDB SQL 教程”。

[
   "Hello World, this is DocumentDB SQL Tutorial"
]

在另一个例子中，我们来看看上面例子中的三个文档。

以下是 AndersenFamily 文档。

{
   "id": "AndersenFamily",
   "lastName": "Andersen",

   "parents": [
      { "firstName": "Thomas", "relationship":  "father" },
      { "firstName": "Mary Kay", "relationship":  "mother" }
   ],

   "children": [
      {
         "firstName": "Henriette Thaulow",
         "gender": "female",
         "grade": 5,
         "pets": [ { "givenName": "Fluffy", "type":  "Rabbit" } ]
      }
   ],

   "location": { "state": "WA", "county": "King", "city": "Seattle" },
   "isRegistered": true
}

以下是 SmithFamily 文档。

{
   "id": "SmithFamily",

   "parents": [
      { "familyName": "Smith", "givenName": "James" },
      { "familyName": "Curtis", "givenName": "Helen" }
   ],

   "children": [
      {
         "givenName": "Michelle",
         "gender": "female",
         "grade": 1
      },

      {
         "givenName": "John",
         "gender": "male",
         "grade": 7,

         "pets": [
            { "givenName": "Tweetie", "type": "Bird" }
         ]
      }
   ],

   "location": {
      "state": "NY",
      "county": "Queens",
      "city": "Forest Hills"
   },

   "isRegistered": true
}

以下是 WakefieldFamily 文档。

{
   "id": "WakefieldFamily",

   "parents": [
      { "familyName": "Wakefield", "givenName": "Robin" },
      { "familyName": "Miller", "givenName": "Ben" }
   ],

   "children": [
      {
         "familyName": "Merriam",
         "givenName": "Jesse",
         "gender": "female",
         "grade": 6,

         "pets": [
            { "givenName": "Charlie Brown", "type": "Dog" },
            { "givenName": "Tiger", "type": "Cat" },
            { "givenName": "Princess", "type": "Cat" }
         ]
      },

      {
         "familyName": "Miller",
         "givenName": "Lisa",
         "gender": "female",
         "grade": 3,

         "pets": [
            { "givenName": "Jake", "type": "Snake" }
         ]
      }
   ],

   "location": { "state": "NY", "county": "Manhattan", "city": "NY" },
   "isRegistered": false
}

以下是查询。

SELECT VALUE f.location
FROM Families f

当执行此查询时，他返回地址而不带位置标签。

[
   {
      "state": "NY",
      "county": "Manhattan",
      "city": "NY"
   },

   {
      "state": "NY",
      "county": "Queens",
      "city": "Forest Hills"
   },

   {
      "state": "WA",
      "county": "King",
      "city": "Seattle"
   }
]

如果我们现在指定相同的查询而无 VALUE 关键字，那么将会返回带有位置标签的地址。以下是查询。

SELECT f.location
FROM Families f

执行此查询时，将得到以下输出。

[
   {
      "location": {
         "state": "NY",
         "county": "Manhattan",
         "city": "NY"
      }
   },

   {
      "location": {
         "state": "NY",
         "county": "Queens",
         "city": "Forest Hills"
      }
   },

   {
      "location": {
         "state": "WA",
         "county": "King",
         "city": "Seattle"
      }
   }
]