LINQ 从多个联接中获取具有最大日期的记录

Question

我有三个表：课程、课程位置、课程时间表

每门课程可以在一个或多个地点举行（一对多） 每个地点可以举办一个或多个时间表（一对多）

我需要获取具有 Schedules.Date> Today 的所有课程（唯一名称），并显示表 CourseSchedules 中包含的日期的最大值

我当前的 linq 代码是：

var courses = (from c in db.Courses
           join cl in db.CourseLocations on c.CourseID equals cl.CourseID
           join cs in db.CourseSchedules on cl.CourseLocationID equals cs.CourseLocationID 

           where c.CourseStatusID == 1 && c.DeleteDate == null && ((c.CourseCategoryID == 1 && cs.EndDate >= courseEndDateFilter) || (c.CourseCategoryID == 3))

           select new
           {
               c.CourseID,
               CourseName = c.Name,
               CourseEndDate = cs.EndDate
           }).Distinct().OrderBy(o => o.CourseCategoryID).ThenBy(o => o.CourseName);

其中 courseEndDateFilter 是用于定义要过滤的日期的变量。

上述查询的问题是我得到了所有重复的课程，而不仅仅是具有 MAX 值 cs.EndDate 的课程

有没有办法（有效）做到这一点？

Answer 1

@Ehsan 是正确的。您需要一个分组依据，然后获取 EndDate 的最大值。给定以下模型：

    public class Course
    {
        public int CourseID { get; set; }
        public string Name { get; set; }
        public int CourseStatusID { get; set; }
        public int CourseCategoryID { get; set; }
        public DateTime? DeleteDate { get; set; }
    }

    public class CourseLocation
    {
        public int CourseLocationID { get; set; }
        public int CourseID { get; set; }
    }

    public class CourseSchedules
    {
        public int CourseLocationID { get; set; }
        public DateTime EndDate { get; set; }
    }

我在内存对象中创建了以下内容：

        var courses = new List<Course>
        {
            new Course { CourseID = 1, Name = "Test1", CourseCategoryID = 1, CourseStatusID = 1, DeleteDate = null },
            new Course { CourseID = 2, Name = "Test2", CourseCategoryID = 1, CourseStatusID = 1, DeleteDate = null },
            new Course { CourseID = 3, Name = "Test3", CourseCategoryID = 3, CourseStatusID = 1, DeleteDate = null }
        };
        var courseLocations = new List<CourseLocation>
        {
            new CourseLocation{ CourseID = 1, CourseLocationID = 1 },
            new CourseLocation{ CourseID = 2, CourseLocationID = 1 },
            new CourseLocation{ CourseID = 3, CourseLocationID = 1 },
            new CourseLocation{ CourseID = 1, CourseLocationID = 2 },
            new CourseLocation{ CourseID = 2, CourseLocationID = 2 },
            new CourseLocation{ CourseID = 3, CourseLocationID = 2 }

        };
        var courseSchedules = new List<CourseSchedules>
        {
            new CourseSchedules { CourseLocationID = 1, EndDate = DateTime.Now.AddDays(10) },
            new CourseSchedules { CourseLocationID = 1, EndDate = DateTime.Now.AddYears(1) }
        };

然后查询将如下以获取 Max EndDate：

var result = (from c in courses
                join cl in courseLocations on c.CourseID equals cl.CourseID
                join cs in courseSchedules on cl.CourseLocationID equals cs.CourseLocationID

                where c.CourseStatusID == 1 && c.DeleteDate == null &&
                      (c.CourseCategoryID == 1 && cs.EndDate >= DateTime.Now || c.CourseCategoryID == 3)

                select new
                {
                    c.CourseID,
                    CourseName = c.Name,
                    CourseEndDate = cs.EndDate,
                    c.CourseCategoryID
                })
            .GroupBy(arg => new
            {
                arg.CourseID,
                arg.CourseName,
                arg.CourseCategoryID
            })
            .Select(grouping => new
            {
                grouping.Key.CourseID,
                grouping.Key.CourseName,
                CourseEndDate = grouping.Max(arg => arg.CourseEndDate),
                grouping.Key.CourseCategoryID
            })
            .OrderBy(o => o.CourseCategoryID)
            .ThenBy(o => o.CourseName);