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检索不同字段值

Overview

在本指南中，您可以学习；了解如何使用.NET/ C#驱动程序检索集合中指定字段的不同值。

在集合中，不同文档的单个字段可能包含不同值。示例，restaurants集合中一个文档的borough 值为 "Manhattan"，而另一文档的 borough 值为 "Queens"。通过使用.NET/ C#驱动程序，您可以检索集合中多个文档中某个字段包含的所有唯一值。

样本数据

本指南中的示例使用Atlas示例数据集中的 sample_restaurants.restaurants集合。要学习；了解如何创建免费的MongoDB Atlas 群集并加载示例数据集，请参阅.NET/ C#驱动程序入门。

本页上的示例使用以下 Restaurant 类对集合中的文档进行建模：

public class Restaurant {
    public ObjectId? Id { get; set; }
    [BsonElement("name")]
    public string? Name { get; set; }
    [BsonElement("cuisine")]
    public string? Cuisine { get; set; }
    [BsonElement("borough")]
    public string? Borough { get; set; }
}

Retrieve Distinct Values

要检索指定字段的非重复值，请调用 IMongoCollection<TDocument>实例的 Distinct() 或 DistinctAsync() 方法，并传递要查找其非重复值的字段的名称。

检索集合中的值

以下示例检索 restaurants集合中 borough字段的非重复值。选择 Synchronous 或 Asynchronous标签页以查看相应的代码。

var results = collection.Distinct<string>(r => r.Borough, Builders<Restaurant>.Filters.Empty).ToList();
foreach (var result in results)
{
    Console.WriteLine(result);
}

Bronx
Brooklyn
Manhattan
Missing
Queens
Staten Island

var results = await collection.DistinctAsync<string>(r => r.Borough, Builders<Restaurant>.Filters.Empty);
await results.ForEachAsync(result => Console.WriteLine(result));

Bronx
Brooklyn
Manhattan
Missing
Queens
Staten Island

该操作会返回一个游标，您可以遍历该游标以访问权限每个不同的 borough字段值。尽管多个文档在 borough字段中具有相同的值，但每个值仅在结果中出现一次。

检索指定文档中的值

您可以为 Distinct() 和 DistinctAsync() 方法提供查询过滤，以在集合的文档子集中查找不同的字段值。查询过滤是一个表达式，用于指定在操作中匹配文档的搜索条件。有关创建查询过滤的更多信息，请参阅“创建查询筛选器”指南。

以下示例检索 cuisine字段值为 "Italian" 的所有文档的 borough字段的非重复值。选择 Synchronous 或 Asynchronous标签页以查看相应的代码。

var filter = Builders<Restaurant>.Filter.Eq(r => r.Cuisine, "Italian");
var results = collection.Distinct<string>(r => r.Borough, filter).ToList();
foreach (var result in results)
{
    Console.WriteLine(result);
}

var filter = Builders<Restaurant>.Filter.Eq(r => r.Cuisine, "Italian");
var results = await collection.DistinctAsync<string>(r => r.Borough, filter);
await results.ForEachAsync(result => Console.WriteLine(result));

修改不同行为

您可以通过提供 DistinctOptions实例作为可选参数来修改 Distinct() 和 DistinctAsync() 方法的行为。下表描述了您可以在 DistinctOptions实例上设立的属性：

方法	说明
`Collation`	Sets the collation to use for the operation. See the Collation section of this page for more information. Default: `null` Data type: Collation
`MaxTime`	设置操作可以运行的最长时间。数据类型：`TimeSpan`
`Comment`	Attaches a comment to the operation. Data type: BsonValue or `string`

以下示例检索 borough字段值为 "Bronx" 且 cuisine字段值为 "Pizza" 的所有文档的 name字段的非重复值。然后，它通过为 Distinct() 方法提供 DistinctOptions实例来为操作添加注释。

选择 Synchronous 或 Asynchronous 标签页，查看相应的代码。

var cuisineFilter = Builders<Restaurant>.Filter.Eq(r => r.Cuisine, "Pizza");
var boroughFilter = Builders<Restaurant>.Filter.Eq(r => r.Borough, "Bronx");
var filter = Builders<Restaurant>.Filter.And(cuisineFilter, boroughFilter);
var options = new DistinctOptions {
    Comment = "Find all Italian restaurants in the Bronx"
};
var results = collection.Distinct<string>(r => r.Name, filter).ToList();
foreach (var result in results)
{
    Console.WriteLine(result);
}

$1.25 Pizza
18 East Gunhill Pizza
2 Bros
Aenos Pizza
Alitalia Pizza Restaurant
Amici Pizza And Pasta
Angie'S Cafe Pizza
...

var cuisineFilter = Builders<Restaurant>.Filter.Eq(r => r.Cuisine, "Pizza");
var boroughFilter = Builders<Restaurant>.Filter.Eq(r => r.Borough, "Bronx");
var filter = Builders<Restaurant>.Filter.And(cuisineFilter, boroughFilter);
var options = new DistinctOptions {
    Comment = "Find all Italian restaurants in the Bronx"
};
var results = await collection.DistinctAsync<string>(r => r.Name, filter, options);
await results.ForEachAsync(result => Console.WriteLine(result));

$1.25 Pizza
18 East Gunhill Pizza
2 Bros
Aenos Pizza
Alitalia Pizza Restaurant
Amici Pizza And Pasta
Angie'S Cafe Pizza
...

排序规则

要为操作配置排序规则，请创建 Collation 类的实例。

下表描述了 Collation 构造函数接受的参数。它还列出了相应的类属性，您可以使用这些属性读取每个设置的值。

Parameter	说明	类属性
`locale`	Specifies the International Components for Unicode (ICU) locale. For a list of supported locales, see Collation Locales and Default Parameters in the MongoDB Server Manual. If you want to use simple binary comparison, use the `Collation.Simple` static property to return a `Collation` object with the `locale` set to `"simple"`. Data Type: `string`	`Locale`
`caseLevel`	（可选）指定是否包含大小写比较。当此参数为 `true` 时，驱动程序的行为取决于 `strength` 参数的值： - 如果 `strength` 为 `CollationStrength.Primary`，则驱动程序会比较基本字符和大小写。 - 如果 `strength` 为 `CollationStrength.Secondary`，则驱动程序会比较基本字符、变音符、其他次要差异和大小写。 - 如果 `strength` 为任何其他值，则会忽略此参数。当此参数为 `false` 时，驱动程序不会在强度级别 `Primary` 或 `Secondary` 包含大小写比较。数据类型：`boolean` 默认值：`false`	`CaseLevel`
`caseFirst`	(Optional) Specifies the sort order of case differences during tertiary level comparisons. Data Type: CollationCaseFirst Default: `CollationCaseFirst.Off`	`CaseFirst`
`strength`	(Optional) Specifies the level of comparison to perform, as defined in the ICU documentation. Data Type: CollationStrength Default: `CollationStrength.Tertiary`	`Strength`
`numericOrdering`	(Optional) Specifies whether the driver compares numeric strings as numbers. If this argument is `true`, the driver compares numeric strings as numbers. For example, when comparing the strings "10" and "2", the driver treats the values as 10 and 2, and finds 10 to be greater. If this argument is `false` or excluded, the driver compares numeric strings as strings. For example, when comparing the strings "10" and "2", the driver compares one character at a time. Because "1" is less than "2", the driver finds "10" to be less than "2". For more information, see Collation Restrictions in the MongoDB Server manual. Data Type: `boolean` Default: `false`	`NumericOrdering`
`alternate`	(Optional) Specifies whether the driver considers whitespace and punctuation as base characters for purposes of comparison. Data Type: CollationAlternate Default: `CollationAlternate.NonIgnorable` (spaces and punctuation are considered base characters)	`Alternate`
`maxVariable`	(Optional) Specifies which characters the driver considers ignorable when the `alternate` argument is `CollationAlternate.Shifted`. Data Type: CollationMaxVariable Default: `CollationMaxVariable.Punctuation` (the driver ignores punctuation and spaces)	`MaxVariable`
`normalization`	(Optional) Specifies whether the driver normalizes text as needed. Most text doesn't require normalization. For more information about normalization, see the ICU documentation. Data Type: `boolean` Default: `false`	`Normalization`
`backwards`	（可选）指定包含变音符号的字符串是否从字符串的后部到前部排序。数据类型：`boolean` 默认：`false`	`Backwards`

有关排序规则的更多信息，请参阅MongoDB Server手册中的排序规则页面。

API 文档

要进一步了解本指南所讨论的任何方法或类型，请参阅以下 API 文档：