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通过 Cloud Firestore 对数据进行排序和限制其数量
使用集合让一切井井有条根据您的偏好保存内容并对其进行分类。

Cloud Firestore 提供了强大的查询功能，用于指定您要从集合中检索哪些文档。这些查询也可以与 get() 或 addSnapshotListener() 结合使用，如获取数据中所述。

对数据进行排序和限制其数量

默认情况下，查询会按文档 ID 以升序检索符合查询条件的所有文档。您可以使用 orderBy() 指定数据的排序顺序，并可使用 limit() 限制检索到的文档数量。如果您指定 limit()，其值必须大于或等于零。

例如，您可以通过以下代码查询按字母顺序排列的前 3 个城市：

Web

import { query, orderBy, limit } from "firebase/firestore";  

const q = query(citiesRef, orderBy("name"), limit(3));order_and_limit.js

Web

citiesRef.orderBy("name").limit(3);test.firestore.js

Swift

注意：此产品不适用于 watchOS 和 App Clip 目标。

citiesRef.order(by: "name").limit(to: 3)ViewController.swift

Objective-C

注意：此产品不适用于 watchOS 和 App Clip 目标。

[[citiesRef queryOrderedByField:@"name"] queryLimitedTo:3];ViewController.m

Kotlin

citiesRef.orderBy("name").limit(3)DocSnippets.kt

Java

citiesRef.orderBy("name").limit(3);DocSnippets.java

Dart

final citiesRef = db.collection("cities");
citiesRef.orderBy("name").limit(3);firestore.dart

Java

Query query = cities.orderBy("name").limit(3);
Query query = cities.orderBy("name").limitToLast(3);QueryDataSnippets.java

Python

cities_ref = db.collection("cities")
query = cities_ref.order_by("name").limit_to_last(2)
results = query.get()snippets.py

Python

cities_ref = db.collection("cities")
query = cities_ref.order_by("name").limit_to_last(2)
results = await query.get()snippets.py

C++

cities_ref.OrderBy("name").Limit(3);snippets.cpp

Node.js

const firstThreeRes = await citiesRef.orderBy('name').limit(3).get();index.js

Go

query := cities.OrderBy("name", firestore.Asc).Limit(3)
query := cities.OrderBy("name", firestore.Asc).LimitToLast(3)query.go

PHP

如需详细了解如何安装和创建 Cloud Firestore 客户端，请参阅 Cloud Firestore 客户端库。

$query = $citiesRef->orderBy('name')->limit(3);query_order_limit.php

Unity

Query query = citiesRef.OrderBy("Name").Limit(3);

C#

Query query = citiesRef.OrderBy("Name").Limit(3);Program.cs

Ruby

query = cities_ref.order("name").limit(3)order_limit_data.rb

您也可以按降序排序以获取最后 3 个城市：

Web

import { query, orderBy, limit } from "firebase/firestore";  

const q = query(citiesRef, orderBy("name", "desc"), limit(3));order_and_limit_desc.js

Web

citiesRef.orderBy("name", "desc").limit(3);test.firestore.js

Swift

注意：此产品不适用于 watchOS 和 App Clip 目标。

citiesRef.order(by: "name", descending: true).limit(to: 3)ViewController.swift

Objective-C

注意：此产品不适用于 watchOS 和 App Clip 目标。

[[citiesRef queryOrderedByField:@"name" descending:YES] queryLimitedTo:3];ViewController.m

Kotlin

citiesRef.orderBy("name", Query.Direction.DESCENDING).limit(3)DocSnippets.kt

Java

citiesRef.orderBy("name", Direction.DESCENDING).limit(3);DocSnippets.java

Dart

final citiesRef = db.collection("cities");
citiesRef.orderBy("name", descending: true).limit(3);firestore.dart

Java

Query query = cities.orderBy("name", Direction.DESCENDING).limit(3);QueryDataSnippets.java

Python

cities_ref = db.collection("cities")
query = cities_ref.order_by("name", direction=firestore.Query.DESCENDING).limit(3)
results = query.stream()snippets.py

Python

cities_ref = db.collection("cities")
query = cities_ref.order_by("name", direction=firestore.Query.DESCENDING).limit(3)
results = query.stream()snippets.py

C++

cities_ref.OrderBy("name", Query::Direction::kDescending).Limit(3);snippets.cpp

Node.js

const lastThreeRes = await citiesRef.orderBy('name', 'desc').limit(3).get();index.js

Go

query := cities.OrderBy("name", firestore.Desc).Limit(3)query.go

PHP

如需详细了解如何安装和创建 Cloud Firestore 客户端，请参阅 Cloud Firestore 客户端库。

$query = $citiesRef->orderBy('name', 'DESC')->limit(3);query_order_desc_limit.php

Unity

Query query = citiesRef.OrderByDescending("Name").Limit(3);

C#

Query query = citiesRef.OrderByDescending("Name").Limit(3);Program.cs

Ruby

query = cities_ref.order("name", "desc").limit(3)order_limit_data.rb

您还可以按多个字段排序。例如，如果您想按州排序，并在每个州按人口数量以降序排序，可使用以下代码：

Web

import { query, orderBy } from "firebase/firestore";  

const q = query(citiesRef, orderBy("state"), orderBy("population", "desc"));order_multiple.js

Web

citiesRef.orderBy("state").orderBy("population", "desc");test.firestore.js

Swift

注意：此产品不适用于 watchOS 和 App Clip 目标。

citiesRef
  .order(by: "state")
  .order(by: "population", descending: true)ViewController.swift

Objective-C

注意：此产品不适用于 watchOS 和 App Clip 目标。

[[citiesRef queryOrderedByField:@"state"] queryOrderedByField:@"population" descending:YES];ViewController.m

Kotlin

citiesRef.orderBy("state").orderBy("population", Query.Direction.DESCENDING)DocSnippets.kt

Java

citiesRef.orderBy("state").orderBy("population", Direction.DESCENDING);DocSnippets.java

Dart

final citiesRef = db.collection("cities");
citiesRef.orderBy("state").orderBy("population", descending: true);firestore.dart

Java

Query query = cities.orderBy("state").orderBy("population", Direction.DESCENDING);QueryDataSnippets.java

Python

cities_ref = db.collection("cities")
ordered_city_ref = cities_ref.order_by("state").order_by(
    "population", direction=firestore.Query.DESCENDING
)snippets.py

Python

cities_ref = db.collection("cities")
cities_ref.order_by("state").order_by(
    "population", direction=firestore.Query.DESCENDING
)snippets.py

C++

cities_ref.OrderBy("state").OrderBy("name", Query::Direction::kDescending);snippets.cpp

Node.js

const byStateByPopRes = await citiesRef.orderBy('state').orderBy('population', 'desc').get();index.js

Go

query := client.Collection("cities").OrderBy("state", firestore.Asc).OrderBy("population", firestore.Desc)query.go

PHP

如需详细了解如何安装和创建 Cloud Firestore 客户端，请参阅 Cloud Firestore 客户端库。

$query = $citiesRef->orderBy('state')->orderBy('population', 'DESC');query_order_multi.php

Unity

Query query = citiesRef.OrderBy("State").OrderByDescending("Population");

C#

Query query = citiesRef.OrderBy("State").OrderByDescending("Population");Program.cs

Ruby

query = cities_ref.order("state").order("population", "desc")order_limit_data.rb

您可以将 where() 过滤条件与 orderBy() 和 limit() 结合使用。下例中的查询定义了一个人口数量阈值，并按人口数量升序显示超过该阈值的前几个结果：

Web

import { query, where, orderBy, limit } from "firebase/firestore";  

const q = query(citiesRef, where("population", ">", 100000), orderBy("population"), limit(2));filter_and_order.js

Web

citiesRef.where("population", ">", 100000).orderBy("population").limit(2);test.firestore.js

Swift

注意：此产品不适用于 watchOS 和 App Clip 目标。

citiesRef
  .whereField("population", isGreaterThan: 100000)
  .order(by: "population")
  .limit(to: 2)ViewController.swift

Objective-C

注意：此产品不适用于 watchOS 和 App Clip 目标。

[[[citiesRef queryWhereField:@"population" isGreaterThan:@100000]
    queryOrderedByField:@"population"]
    queryLimitedTo:2];ViewController.m

Kotlin

citiesRef.whereGreaterThan("population", 100000).orderBy("population").limit(2)DocSnippets.kt

Java

citiesRef.whereGreaterThan("population", 100000).orderBy("population").limit(2);DocSnippets.java

Dart

final citiesRef = db.collection("cities");
citiesRef
    .where("population", isGreaterThan: 100000)
    .orderBy("population")
    .limit(2);firestore.dart

Java

Query query = cities.whereGreaterThan("population", 2500000L).orderBy("population").limit(2);QueryDataSnippets.java

Python

cities_ref = db.collection("cities")
query = (
    cities_ref.where(filter=FieldFilter("population", ">", 2500000))
    .order_by("population")
    .limit(2)
)
results = query.stream()snippets.py

Python

cities_ref = db.collection("cities")
query = (
    cities_ref.where(filter=FieldFilter("population", ">", 2500000))
    .order_by("population")
    .limit(2)
)
results = query.stream()snippets.py

C++

cities_ref.WhereGreaterThan("population", FieldValue::Integer(100000))
    .OrderBy("population")
    .Limit(2);snippets.cpp

Node.js

const biggestRes = await citiesRef.where('population', '>', 2500000)
  .orderBy('population').limit(2).get();index.js

Go

query := cities.Where("population", ">", 2500000).OrderBy("population", firestore.Desc).Limit(2)query.go

PHP

如需详细了解如何安装和创建 Cloud Firestore 客户端，请参阅 Cloud Firestore 客户端库。

$query = $citiesRef
    ->where('population', '>', 2500000)
    ->orderBy('population')
    ->limit(2);query_order_limit_field_valid.php

Unity

Query query = citiesRef
    .WhereGreaterThan("Population", 2500000)
    .OrderBy("Population")
    .Limit(2);

C#

Query query = citiesRef
    .WhereGreaterThan("Population", 2500000)
    .OrderBy("Population")
    .Limit(2);Program.cs

Ruby

query = cities_ref.where("population", ">", 2_500_000).order("population").limit(2)order_limit_data.rb

但是，如果您的过滤条件包含范围比较运算符（<、<=、>、>=），您的第一重排序必须针对同一个字段，请参阅下面的 orderBy() 限制列表。

限制

请注意，orderBy() 子句有以下限制：

orderBy() 子句也会根据是否存在给定字段进行过滤。不含给定字段的文档将不会包含在结果集中。

`orderBy` 及其存在性

当您按给定字段对查询进行排序时，查询只会返回存在排序依据 (order-by) 字段的文档。

例如，以下查询不会返回任何未设置 population 字段的文档，即使它们符合查询过滤条件也是如此。

Java

db.collection("cities").whereEqualTo("country", “USA”).orderBy(“population”);

类似的规则也适用于不等式。如果查询对某个字段使用不等式过滤条件，则系统会按该字段进行排序。以下查询不会返回不含 population 字段的文档，即使该文档中有 country = USA 也是如此。如需解决此问题，您可以为每个排序执行单独的查询，也可以为所有排序依据字段分配一个值。

Java

db.collection(“cities”).where(or(“country”, USA”), greaterThan(“population”, 250000));

上面查询中的不等式隐含了排序依据 (order-by)，它等效于下面的查询：

Java

db.collection(“cities”).where(or(“country”, USA”), greaterThan(“population”, 250000)).orderBy(“population”);

通过 Cloud Firestore 对数据进行排序和限制其数量 使用集合让一切井井有条 根据您的偏好保存内容并对其进行分类。

对数据进行排序和限制其数量

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限制

orderBy 及其存在性

Java

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Java

通过 Cloud Firestore 对数据进行排序和限制其数量
使用集合让一切井井有条根据您的偏好保存内容并对其进行分类。

`orderBy` 及其存在性