Question

我使用左连接并使用group by和order by在linq中编写了此查询。我有2张桌子，学生和课程。所以我在学习桌上分组后，我没有在课程中获得任何专栏，所以如何在datetime专栏的课程表上订购。

tablestudent：

id，name，std

tablecourse：

ID，用户ID，日期时间

这是我的疑问：

 var res = (from u in db.student
              join c in db.course on u.id equals c.userid into j1
              from j2 in j1.DefaultIfEmpty()                       
              group j2 by u.id into g
              orderby g.Key.datatime descending    // here i'm getting datetime does not contain.
              select new { id = g.key }).tolist(); // here i just want id not getting any other column.

如果有人知道该怎么做，请帮助。

Answer 1

这个怎么样

   var query =db.course.GroupBy(c => c.UserId)
             .Select(g => g.OrderByDescending(c => c.datetime).FirstOrDefault());
        var res = (from u in db.student
              join c in query on u.id equals c.userid into j1
              from j2 in j1.DefaultIfEmpty()     
              select new {   u.name,j2}).ToList();

Working fiddle

Linq使用如何使用order by和group by with left join？

1 个答案: