DocumentDB SQL - 值关键字

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

让我们看一个简单的例子。

以下是使用 VALUE 关键字的查询。

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

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

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

在另一个示例中，让我们考虑前面示例中的三个文档。

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 
}

以下是史密斯家族的文件。

{ 
   "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

执行此查询时，它返回不带位置标签的地址。

[ 
   { 
      "state": "NY", 
      "county": "Manhattan", 
      "city": "NY" 
   }, 
	
   { 
      "state": "NY", 
      "county": "Queens", 
      "city": "Forest Hills" 
   },
	
   { 
      "state": "WA", 
      "county": "King", 
      "city": "Seattle" 
   } 
] 

如果我们现在指定不带 VALUE 关键字的相同查询，那么它将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，它会产生以下输出。

[ 
   { 
      "location": { 
         "state": "NY", 
         "county": "Manhattan", 
         "city": "NY" 
      } 
   }, 
	
   { 
      "location": { 
         "state": "NY", 
         "county": "Queens", 
         "city": "Forest Hills" 
      } 
   },
	
   { 
      "location": { 
         "state": "WA", 
         "county": "King", 
         "city": "Seattle" 
      } 
   } 
]