Question

如果我有这3个班级：

class Student { int id; string name; }
class Course { int id; string name; }
class Enrolment { int studentId; int courseId; DateTime enrolmentDate; }

然后用这个：

IEnumerable<Course> FindCoursesForStudent(Student student)
{
    return from enrolment in Enrolments 
           where enrolment.studentId == student.id
           join course in Courses
              on enrolment.courseId equals course.id
           select course;
}

我可以列出学生注册的所有课程。

但如果学生不止一次注册课程，我怎么才会显示每门课程的最新注册？

我假设我需要按课程分组并过滤enrolmentDate = Max（enrolmentDate）或类似内容？

（如果这看起来很熟悉，那是因为它是我previous LINQ question的后续内容。）

Answer 1

Enumerable<Course> FindCoursesForStudent(Student student)
{
    return from enrolment in Enrolments 
           where enrolment.studentId == student.id
           && enrolment.enrolmentDate == (from ed in Enrolments 
           where ed.enrolmentDate == enrolment.enrolmentDate 
           select ed.enrolmentDate).Max()
           join course in Courses
              on enrolment.courseId equals course.id
           select course;
}

Answer 2

要获得学生每门课程的最新报名，我会使用以下内容：

    IEnumerable<Enrolment> FindCoursesForStudent(Student student) {
        return from enrolment in Enrolments
               where enrolment.studentId == student.id
               group enrolment by enrolment.courseId into gp
               select gp.OrderBy(e => e.enrolmentDate).First();
    }

这就是你要求的吗？

LINQ - 分组和聚合

2 个答案: