Drill 18: Compare — LINQ in depth

record Course(int Id, string Name);
record Lesson(int CourseId, string Title, int Minutes);

var courses = new[]
{
    new Course(1, "C#"),
    new Course(2, "C"),
};

var lessons = new[]
{
    new Lesson(1, "Variables", 12),
    new Lesson(1, "Methods", 11),
    new Lesson(2, "Pointers", 15),
    new Lesson(2, "Structs", 14),
};

var query = lessons.Where(lesson => lesson.Minutes > 11);

var flat = courses
    .SelectMany(course => lessons.Where(lesson => lesson.CourseId == course.Id))
    .ToList();

var grouped = lessons
    .GroupBy(lesson => lesson.CourseId)
    .Select(group => new { CourseId = group.Key, Count = group.Count() })
    .ToList();

var joined = courses
    .Join(lessons,
        course => course.Id,
        lesson => lesson.CourseId,
        (course, lesson) => course.Name + ": " + lesson.Title)
    .ToList();

var materialised = query.ToList();