Question

我对LINQ相对较新，但是对SQL本身很精通。我有一个查询，试图在其中加入2个表-一个表是汽车列表，另一个表是每个表的测试记录。我想出了如何加入最新测试的方法，但没有获得特定于此的评论字段。

所以可以说测试数据是：

CarID   |TestDriveDate          |Comments
========+=======================+===========
1       |1/7/2019               |ok
2       |12/7/2019              |average
2       |10/7/2019              |poor
2       |15/7/2019              |good

使用以下代码：

var query = from test in TestDrives
                  group test by test.CarID into g
                  join car in Cars
                  on g.Key equals car.CarID

                  select new TestViewModel
                  {
                      CarID = g.Key,
                      CarName = car.CarName
                      Model = car.Model
                      LastTestDrive = g.Max(a => a.TestDriveDate),
                      //and the comment field where its the max inspection date
                      Comments = g.Select(a => a.Comments).FirstOrDefault()
                  };

因此应返回以下记录：

CarID   |TestDriveDate          |Comments
========+=======================+===========
2       |15/7/2019              |good

Answer 1

您不需要更改Group，只需添加Sort。

Comments = g.Select(a => a.Comments).Sort((test1, test2)=>test1.TestDriveDate.CompareTo(test2.TestDriveDate)).FirstOrDefault();

或将orderby添加到select。

var query = from test in TestDrives
                  orderby test.TestDriveDate
                  group test by test.CarID into g
                  join car in Cars
                  on g.Key equals car.CarID

按最近日期分组

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