在本章中，我们将介绍FROM子句，它与常规SQL中的标准FROM子句不同.
查询始终在特定集合的上下文中运行，无法加入跨集合中的文档，这让我们想知道为什么我们需要一个FROM子句.事实上，我们没有，但如果我们不包含它，那么我们就不会在集合中查询文档.
此子句的目的是指定数据查询必须运行的源.通常整个集合是源，但可以指定集合的子集. FROM< from_specification>除非在查询中稍后过滤或投影源，否则子句是可选的.
让我们再看一下相同的例子.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
}
在上面的查询中，" SELECT * FROM c "表示整个家庭col lection是枚举的源.
源也可以缩减为更小的子集.当我们只想在每个文档中检索一个子树时，子根可以成为源，如下例所示.
当我们运行以下查询 :
SELECT * FROM Families.parents
将检索以下子文档.
[
[
{
"familyName": "Wakefield",
"givenName": "Robin"
},
{
"familyName": "Miller",
"givenName": "Ben"
}
],
[
{
"familyName": "Smith",
"givenName": "James"
},
{
"familyName": "Curtis",
"givenName": "Helen"
}
],
[
{
"firstName": "Thomas",
"relationship": "father"
},
{
"firstName": "Mary Kay",
"relationship": "mother"
}
]
]由于此查询，我们可以看到只检索父母子文档.
