C# LINQ

Lnaguage Intergrated Query(Linq) 集成查詢語言
基本查詢表達式模式來查詢和轉換 SQL 數據庫、ADO.NET 數據集、XML 文檔和流以及 .NET 集合中的數據

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Linq語句的三個步驟：
1.定義數據源；
2.定義查詢語句；
3.執行查詢語句。

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sample1

class LINQQueryExpressions
{
    static void Main()
    {
        // 定義數據源
        int[] scores = new int[] { 97, 92, 81, 60 };

        // 定義查詢語句
        IEnumerable scoreQuery =
            from score in scores
            where score > 80
            select score;

        // 執行查詢語句
        foreach (int i in scoreQuery)
        {
            Console.Write(i + " ");
        }
        //執行query語句的其他方法
        //scoreQuery.ToList();
        //scoreQuery.ToArray();
    }
}

query語句在沒有使用到這個數據集的時候，是沒有執行的，這里需要注意數據調用的先后順序。以免造成錯誤。

查詢兩種方法

query語句形式，方法的形式

query語句形式如sample1所示。
方法的形式

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

在上面的查詢中，用到的是int型數據這里轉換到string類型，代碼如下：

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

由於查詢存儲了結果，已經是一個元素集，在程序中不能將查詢變量單純看為一個變量。

select,group,into關鍵字

// 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子句集合之間的連接，左連接，右連接。

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 + " ");

子查詢

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()
    };