DocumentDB SQL - 聯接

在關係資料庫中，Joins 子句用於組合來自資料庫中兩個或多個表的記錄，並且在設計規範化模式時，跨表聯接的需求非常重要。由於 DocumentDB 處理無模式文件的非規範化資料模型，因此 DocumentDB SQL 中的 JOIN 等同於“自聯接”。

讓我們考慮前面示例中的三個文件。

以下是AndersenFamily 文件。

{ 
   "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 文件。

{ 
   "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 
} 

讓我們看一個例子來了解 JOIN 子句是如何工作的。

以下是將根與子文件 children 聯接的查詢。

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

執行上述查詢後，將產生以下輸出。

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

在上面的示例中，聯接是在文件根和子根 children 之間進行的，它在兩個 JSON 物件之間進行笛卡爾積。以下是一些需要注意的事項：

• 在 FROM 子句中，JOIN 子句是一個迭代器。

• 前兩個文件 WakefieldFamily 和 SmithFamily 包含兩個子項，因此結果集也包含笛卡爾積，它為每個子項生成一個單獨的物件。

• 第三個文件 AndersenFamily 只包含一個子項，因此此文件只有一個對應物件。

讓我們再看一個相同的例子，但是這次我們也檢索子項名稱，以便更好地理解 JOIN 子句。

以下是將根與子文件 children 聯接的查詢。

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

執行上述查詢後，將產生以下輸出。

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