Question

在此列表中，教师可以拥有多个SchoolEndYears。我想修改这个查询，所以我只抓住每个老师的最大值。

TeacherA - 2012
TeacherA - 2013
TeacherA - 2014
TeacherB - 2013
TeacherC - 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);

正如您所看到的，我尝试了.OrderByDescending，但当我尝试.FirstOrDefault时，它只会带回群组中的第一条记录。

我希望带回来的是：

TeacherA - 2014
TeacherB - 2013
TeacherC - 2012

Answer 1

我认为你想要的东西如下，虽然我不确定这是多么有效：

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

您需要的是将同一位教师的记录组合在一起，然后应用Max函数，以便您找到该组的最大元素：

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

为每个项目带回最大结果

2 个答案: