Entity Framework 使用 Left Join

var query = (from s in ctx.Students.Include("ClassRooms")
                         join sd in ctx.StudentDescriptions on s.StudentID equals sd.StudentID into g
                         from stuDesc in g.DefaultIfEmpty()
                         select new
                                    {
                                        Name=s.StudentName,
                                        StudentId=s.StudentID,

                         }).SingleOrDefault();

重点是：on s.StudentID equals sd.StudentID into g

如果出现 NULL的情况，可以使用 Name=(s.StudentName== null ? "" :s.StudentName)