Question

我有以下LINQ查询，但是我想修改它想要由staffId进行分组，并且只选择那些其ObservationDate为每个staffId的Max的记录。

from ob in db.TDTObservations.OfType<TDTSpeedObservation>()
select new
{
   Id = ob.ID,
   AcademicYearId = ob.Teachers.FirstOrDefault().Classes.FirstOrDefault().AcademicYearID,
   observationDate = ob.ObservationDate,
   schoolId = ob.Teachers.FirstOrDefault().Classes.FirstOrDefault().SchoolID,
   staffId=ob.Teachers.FirstOrDefault().ID
};

Answer 1

var observations = 
    from ob in db.TDTObservations.OfType<TDTSpeedObservation>()
    select new  {
       Id = ob.ID,
       AcademicYearId = ob.Teachers.FirstOrDefault().Classes.FirstOrDefault().AcademicYearID,
       observationDate = ob.ObservationDate,
       schoolId = ob.Teachers.FirstOrDefault().Classes.FirstOrDefault().SchoolID,
       staffId=ob.Teachers.FirstOrDefault().ID
    };

var result = from o in observations
             group o by o.staffId into g
             select g.OrderByDescending(x => x.observationDate).First();

Answer 2

怎么样：特此你首先按照他们的ID将你的参赛作品（教师）分组，然后从每个小组（grp）按照最新的ObservationDate选择那个

var observations = from d in db.TDTObservations.OfType<TDTSpeedObservation>()
                   group d by d.Teachers.FirstOrDefault().ID into grp
                   select grp.OrderByDescending(g => g.ObservationDate).FirstOrDefault();

单个查询中的Group By和Max的LINQ查询

2 个答案: