SQL 联接
在关系数据库中，Joins 子句用于合并来自数据库中两个或多个表的记录，在设计规范化模式时，跨表联接的需求非常重要。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
}
让我们看一个例子来了解 JOIN 子句的工作原理。
以下是将根连接到子文档的查询。
SELECT f.id
FROM Families f
JOIN c IN f.children
执行上述查询时，将产生以下输出。
[
{
"id": "WakefieldFamily"
},
{
"id": "WakefieldFamily"
},
{
"id": "SmithFamily"
},
{
"id": "SmithFamily"
},
{
"id": "AndersenFamily"
}
]
在上面的示例中，连接位于文档根和子根之间，这在两个 JSON 对象之间形成交叉产品。以下是一些注意事项-
在 FROM 子句中，JOIN 子句是一个迭代器。
前两个文档 WakefieldFamily 和 SmithFamily 包含两个子项，因此结果集还包含为每个子项生成一个单独对象的叉积。
第三个文档 AndersenFamily 只包含一个孩子，因此该文档对应的对象只有一个。
让我们看一下同一个例子，不过这次我们也检索了孩子的名字，以便更好地理解 JOIN 子句。
以下是将根连接到子文档的查询。
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"
}
]
