选择所有列，但仅在linq中将其分组

问题描述

我一直在寻找一种获取多列的方法，但是在SQL中仅按一个列进行分组，我发现了一些信息.但是我无法在linq中想出一种方法.

I have been looking for a way to get multiple columns but group by only one in SQL and I found some info. However I can not came up with a way to do it in linq.

我有以下玩具示例表:

| Id | Message | GroupId | Date |
|-------------------------------|
| 1  | Hello   | 1       | 1:00 |
| 2  | Hello   | 1       | 1:01 |
| 3  | Hey     | 2       | 2:00 |
| 4  | Dude    | 3       | 3:00 |
| 5  | Dude    | 3       | 3:01 |

我想恢复具有不同 GroupId 的行的所有列，如下所示(以'Date'desc顺序):

And I would like to recover all columns for the rows that have a distinct GroupId as follows (with a 'Date' desc order):

| Id | Message | GroupId | Date |
|-------------------------------|
| 1  | Hello   | 1       | 1:00 |
| 3  | Hey     | 2       | 2:00 |
| 4  | Dude    | 3       | 3:00 |

我真的不在乎从分组的行(第一，第二...)中选择哪一行，只要是给定该组ID的唯一行即可.

I do not really care about which row is picked from the grouped ones (first, second...) as long as is the only one given that group Id.

到目前为止，我已经给出了以下代码，但是它没有执行应做的事情:

I have came out with the following code so far but it does not do what is supposed to:

List<XXX> messages = <MyRep>.Get(<MyWhere>)
                            .GroupBy(x => x.GroupId)
                            .Select(grp => grp.OrderBy(x => x.Date))
                            .OrderBy(y => y.First().Date)
                            .SelectMany(y => y).ToList();

推荐答案

这将为您提供每组一项:

This will give you one item per group:

List<dynamic> data = new List<dynamic>
{
    new {ID  = 1, Message = "Hello", GroupId = 1, Date = DateTime.Now},
    new {ID  = 2, Message = "Hello", GroupId = 1, Date = DateTime.Now},
    new {ID  = 3, Message = "Hey",   GroupId = 2, Date = DateTime.Now},
    new {ID  = 4, Message = "Dude",  GroupId = 3, Date = DateTime.Now},
    new {ID  = 5, Message = "Dude",  GroupId = 3, Date = DateTime.Now},
};

var result = data.GroupBy(item => item.GroupId)
                 .Select(grouping => grouping.FirstOrDefault())
                 .OrderByDescending(item => item.Date)
                 .ToList();

//Or you can also do like this:
var result = data.GroupBy(item => item.GroupId)
                 .SelectMany(grouping => grouping.Take(1))
                 .OrderByDescending(item => item.Date)
                 .ToList();

如果您想控制 OrderBy ，则:

var result = data.GroupBy(item => item.GroupId)
                 .SelectMany(grouping => grouping.OrderBy(item => item.Date).Take(1))
                 .OrderByDescending(item => item.Date)
                 .ToList();

这篇关于选择所有列，但仅在linq中将其分组的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！