带回每个项目的最大结果

Question

在此列表中，一位教师可以有多个 SchoolEndYears。我想修改这个查询，所以我只为每个老师获取最大值。

• A老师 - 2012
• A老师 - 2013
• A 老师 - 2014 年
• B 老师 - 2013 年
• 教师 C - 2012

代码：

var    gridInfo = (from teacher in TeacherAccesses
                   join u in USERS
                   on teacher.ID equals u.ID
                   select new 
                   {
                      Name = u.LastName + ", " + u.FirstName,
                      ID = teacher.ID,
                      Flag = "Existing",
                      SchoolID = teacher.SchoolID,
                      status = teacher.TeacherAttr.IncludeinOfficialResults.Value,
                      SchoolEndYear = teacher.TeacherAttr.SchoolEndYear
                    }).OrderByDescending(x=>x.SchoolEndYear).OrderBy(x=>x.Name);

如您所见，我尝试执行 .OrderByDescending，但当我尝试执行 .FirstOrDefault 时，它只带回了组中的第一条记录。

我希望带回来的是：

• A 老师 - 2014 年
• B 老师 - 2013 年
• 教师 C - 2012

Answer 1

我认为您需要类似以下的内容，尽管我不确定这样做的效率如何：

var gridInfo = (from teacher in TeacherAccesses
                join u in USERS
                on teacher.ID equals u.ID
                where teacher.Year = TeacherAccesses.Where(x => x.ID = teacher.ID)
                                         .Max(x => x.TeacherAttr.SchoolEndYear)
                select new 
                {
                   Name = u.LastName + ", " + u.FirstName,
                   ID = teacher.ID,
                   Flag = "Existing",
                   SchoolID = teacher.SchoolID,
                   status = teacher.TeacherAttr.IncludeinOfficialResults.Value,
                   SchoolEndYear = teacher.TeacherAttr.SchoolEndYear
                 });

这是一个简化的 LINQPad 示例：

void Main()
{
    var TeacherAccesses = new List<Teacher>() {
        new Teacher() { ID = 1, Name = "Teacher-A", TeacherAttr = new Attr() { SchoolEndYear = 2012 } },
        new Teacher() { ID = 1, Name = "Teacher-A", TeacherAttr = new Attr() { SchoolEndYear = 2013 } },
        new Teacher() { ID = 1, Name = "Teacher-A", TeacherAttr = new Attr() { SchoolEndYear = 2014 } },
        new Teacher() { ID = 2, Name = "Teacher-B", TeacherAttr = new Attr() { SchoolEndYear = 2012 } },
        new Teacher() { ID = 2, Name = "Teacher-B", TeacherAttr = new Attr() { SchoolEndYear = 2013 } },
        new Teacher() { ID = 3, Name = "Teacher-C", TeacherAttr = new Attr() { SchoolEndYear = 2012 } }
    };
    //TeacherAccesses.Dump();
    var latest =
        from teacher in TeacherAccesses
        where teacher.TeacherAttr.SchoolEndYear == TeacherAccesses.Where(x => x.ID == teacher.ID).Max(x => x.TeacherAttr.SchoolEndYear)
        select new {
            Name = teacher.Name,
            Year = teacher.TeacherAttr.SchoolEndYear
        };
    latest.Dump();
}

// Define other methods and classes here
public class Teacher
{
    public int ID { get; set; }
    public string Name { get; set; }
    public Attr TeacherAttr { get; set; }
}

public class Attr
{
    public int SchoolEndYear { get; set; }
}

结果：

Teacher-A    2014
Teacher-B    2013
Teacher-C    2012

Answer 2

您需要将同一位老师的记录分组在一起，然后应用Max 函数，它可以让您找到组的最大元素：

var gridInfo = (from teacher in TeacherAccesses
                ...
               })
               .GroupBy(x => x.ID)
               .Select(g => g.Max(x => x.SchoolEndYear))
               .OrderBy(x => x.Name);