C# LINQ

Lnaguage Intergrated Query(Linq) 集成查询语言
基本查询表达式模式来查询和转换 SQL 数据库、ADO.NET 数据集、XML 文档和流以及 .NET 集合中的数据

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Linq语句的三个步骤：
1.定义数据源；
2.定义查询语句；
3.执行查询语句。

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sample1

class LINQQueryExpressions
{
    static void Main()
    {
        // 定义数据源
        int[] scores = new int[] { 97, 92, 81, 60 };

        // 定义查询语句
        IEnumerable scoreQuery =
            from score in scores
            where score > 80
            select score;

        // 执行查询语句
        foreach (int i in scoreQuery)
        {
            Console.Write(i + " ");
        }
        //执行query语句的其他方法
        //scoreQuery.ToList();
        //scoreQuery.ToArray();
    }
}

query语句在没有使用到这个数据集的时候，是没有执行的，这里需要注意数据调用的先后顺序。以免造成错误。

查询两种方法

query语句形式，方法的形式

query语句形式如sample1所示。
方法的形式

var ScoreQuery2=Scores where(score=>score>80) 
                       order by(score=>score);
                       

在上面的查询中，用到的是int型数据这里转换到string类型，代码如下：

IEnumerable highScoresQuery2 =
    from score in scores
    where score > 80
    orderby score descending
    select String.Format("The score is {0}", score);

由于查询存储了结果，已经是一个元素集，在程序中不能将查询变量单纯看为一个变量。

select,group,into关键字

// percentileQuery is an IEnumerable>
var percentileQuery =
    from country in countries
    let percentile = (int) country.Population / 10000000
    group country by percentile into countryGroup
    where countryGroup.Key >= 20
    orderby countryGroup.Key
    select countryGroup;

// grouping is an IGrouping
foreach (var grouping in percentileQuery)
{
    Console.WriteLine(grouping.Key);
    foreach (var country in grouping)
        Console.WriteLine(country.Name + ":" + country.Population);
}

where子句

IEnumerable queryCityPop =
    from city in cities
    where city.Population < 200000 && city.Population > 100000
    select city;

orderby 子句

IEnumerable querySortedCountries =
    from country in countries
    orderby country.Area, country.Population descending
    select country;

Join子句集合之间的连接，左连接，右连接。

var categoryQuery =
    from cat in categories
    join prod in products on cat equals prod.Category
    select new { Category = cat, Name = prod.Name };

let子句

string[] names = { "Svetlana Omelchenko", "Claire O'Donnell", "Sven Mortensen", "Cesar Garcia" };
IEnumerable queryFirstNames =
    from name in names
    let firstName = name.Split(new char[] { ' ' })[0]
    select firstName;

foreach (string s in queryFirstNames)
    Console.Write(s + " ");

子查询

var queryGroupMax =
    from student in students
    group student by student.GradeLevel into studentGroup
    select new
    {
        Level = studentGroup.Key,
        HighestScore =
            (from student2 in studentGroup
             select student2.Scores.Average())
             .Max()
    };