Query Expression Basics (C# Programming Guide)

http://msdn.microsoft.com/ru-ru/library/bb384065.aspx

IEnumerable<int> highScoresQuery =
    from score in scores
    where score > 80
    orderby score descending
    select score;

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

int highScoreCount =
    (from score in scores
     where score > 80
     select score)
     .Count();
/Query syntax
IEnumerable<City> queryMajorCities =
    from city in cities
    where city.Population > 100000
    select city;


// Method-based syntax
IEnumerable<City> queryMajorCities2 = cities.Where(c => c.Population > 100000);

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

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


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

foreach (string s in queryFirstNames)
    Console.Write(s + " ");
//Output: Svetlana Claire Sven Cesar





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()
    };