按最大日期按降序排序，然后按日期升序按组中的单行排序，使用 linq 方法 sintax

Question

I have a table like this

Id      Date       GroupId   Text    Column1   Column2 ...
1    2020-02-02       1      ....     ....       ...
2    2020-02-04       1      ....     ....       ...
3    2020-02-03       1      ....     ....       ...
4    2020-02-02       2      ....     ....       ...
5    2020-02-05       2      ....     ....       ...

I need to obtain this result:

Id      Date       GroupId   Text    Column1   Column2 ...
5    2020-02-05       2      ....     ....       ...
4    2020-02-02       2      ....     ....       ...
1    2020-02-02       1      ....     ....       ...
3    2020-02-03       1      ....     ....       ...
2    2020-02-04       1      ....     ....       ...

I explain I need to get before all rows of the group 2 because the max date is in the group 2... I need to order the group by date descending but every single group should be ordered by date ascending.

I find difficult to write it in sql too. Can anyone help me please? Thank you

EDIT: Here what I have tried to do:

var result = messages.Select(m => new MyViewModel()
            {
                Id = m.Id,
                Date = m.Date,
                Text = m.Text,                    
                GroupId = m.GroupId
            }).GroupBy(d => d.GroupId)
              .SelectMany(g => g)
              .OrderByDescending(x => x.GroupId)
              .ThenBy(x => x.Date);

But it does not work

Answer 1

I suggest creating an intermediate result having a GroupDate property. In my example I am using C# 7.0 Tuples . You could use anonymous types as well, if you are using an older C# version.

var result = messages
    .GroupBy(m => m.GroupId)                                // Create Tuple ...
    .Select(g => (Group: g, GroupDate: g.Max(m => m.Date))) // ... with group and group date.
    .SelectMany(a =>              // Expand the group
        a.Group.Select(m =>       // Create new tuple with group date and model.
            (a.GroupDate,
             Model: new MyViewModel() {
                 Id = m.Id,
                 Date = m.Date,
                 Text = m.Text,
                 GroupId = m.GroupId
             })))
    .OrderByDescending(x => x.GroupDate)
    .ThenBy(x => x.Model.Date)
    .Select(x => x.Model);         // Extract the view model from the tuple.

Result:

Id = 2, Date = 2020-02-02, GroupId = 2
Id = 2, Date = 2020-02-05, GroupId = 2
Id = 1, Date = 2020-02-02, GroupId = 1
Id = 1, Date = 2020-02-03, GroupId = 1
Id = 1, Date = 2020-02-04, GroupId = 1

Example for tuple:

var t = (X: 15, Name: "axis");
Print($"X = {t.X}, Name = {t.Name}");

The name of a tuple property can also be inferred like in (a.GroupDate, Model: ...) . The first property will automatically be called GroupDate . The second is named explicitly Model .

Answer 2

In SQL, you can use window functions:

order by 
    max(date) over(partition by groupId),
    case when max(date) over(partition by groupId) = max(date) over() then date end desc,
    date

Answer 3

You want to order the groups by their max date. Then within each group, order by the date descending.

I would recommend these keys for the order by :

1. maximum date for each group (desc)
2. groupId to keep each group together (order doesn't matter)
3. flag to put the overall maximum date first
4. date for all other rows (asc)

So:

order by max(date) over (partition by groupId) desc, 
         groupId,   -- put each group together
         (case when date = max(date) over() then 1 else 2 end),
         date