在关系数据库中，Joins子句用于组合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处理无模式文档的非规范化数据模型，因此DocumentDB SQL中的JOIN是"selfjoin"的逻辑等价物.
让我们考虑前面示例中的三个文档.
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
}以下是取值mithFamily 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
}让我们看一个例子来了解JOIN子句的工作原理.
以下是将根连接到子子文档的查询.
SELECT f.id FROM Families f JOIN c IN f.children
执行上述查询时，它将产生以下输出.
[
{
"id": "WakefieldFamily"
},
{
"id": "WakefieldFamily"
},
{
"id": "SmithFamily"
},
{
"id": "SmithFamily"
},
{
"id": "AndersenFamily"
}
]在上面的例子中，联接在文档根目录和子子根之间进行交叉两个JSON对象之间的s-product.以下是需要注意和减去的特定要点;
在FROM子句中，JOIN子句是一个迭代器.
前两个文档WakefieldFamily和SmithFamily包含两个子节点，因此结果集还包含为每个子节点生成单独对象的交叉产品.
第三个文档AndersenFamily只包含一个子文档，因此只有一个对象与此文档对应.
让我们看看同一个例子，但这次我们检索子名称以便更好地理解JOIN子句.
以下是将根连接到子子文档的查询.
SELECT f.id AS familyName, c.givenName AS childGivenName, c.firstName AS childFirstName FROM Families f JOIN c IN f.children
执行上述查询时，会产生以下输出.
[
{
"familyName": "WakefieldFamily",
"childGivenName": "Jesse"
},
{
"familyName": "WakefieldFamily",
"childGivenName": "Lisa"
},
{
"familyName": "SmithFamily",
"childGivenName": "Michelle"
},
{
"familyName": "SmithFamily",
"childGivenName": "John"
},
{
"familyName": "AndersenFamily",
"childFirstName": "Henriette Thaulow"
}
]
