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📜  DocumentDB SQL-联接

📅  最后修改于: 2020-11-28 13:36:37             🧑  作者: Mango

在关系数据库中，Joins子句用于合并数据库中两个或多个表的记录，在设计规范化模式时，跨表连接的需求非常重要。由于DocumentDB处理的是无模式文档的非规范化数据模型，因此DocumentDB SQL中的JOIN在逻辑上等同于“自连接”。

让我们像前面的示例一样考虑这三个文档。

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 
}

让我们看一个示例，以了解JOIN子句如何工作。

SQL联接

以下是将根连接到子级子文档的查询。

SELECT f.id 
FROM Families f 
JOIN c IN f.children

执行上述查询后，将产生以下输出。

[ 
   { 
      "id": "WakefieldFamily" 
   },
    
   { 
      "id": "WakefieldFamily" 
   },
    
   { 
      "id": "SmithFamily" 
   },
    
   { 
      "id": "SmithFamily" 
   },
    
   { 
      "id": "AndersenFamily" 
   } 
]

在上面的示例中，联接位于文档根目录和子级子根目录之间，这在两个JSON对象之间形成了叉积。以下是要注意的某些要点-

  • 在FROM子句中，JOIN子句是迭代器。

  • 前两个文档WakefieldFamily和SmithFamily包含两个孩子，因此结果集还包含叉积，该叉积为每个孩子产生一个单独的对象。

  • 第三个文档AndersenFamily仅包含一个孩子，因此该文档仅对应一个对象。

让我们看一下相同的示例，但是这次我们也检索子名称以更好地理解JOIN子句。

SQL联接

以下是将根连接到子级子文档的查询。

SELECT  
   f.id AS familyName, 
   c.givenName AS childGivenName, 
   c.firstName AS childFirstName 
FROM Families f  
JOIN c IN f.children

执行上述查询后，将产生以下输出。

[ 
   { 
      "familyName": "WakefieldFamily", 
      "childGivenName": "Jesse" 
   },
    
   { 
      "familyName": "WakefieldFamily", 
      "childGivenName": "Lisa" 
   },
    
   { 
      "familyName": "SmithFamily", 
      "childGivenName": "Michelle" 
   },
    
   { 
      "familyName": "SmithFamily", 
      "childGivenName": "John" 
   },
    
   { 
      "familyName": "AndersenFamily", 
      "childFirstName": "Henriette Thaulow" 
   } 
]