在LINQ中分组问题

Question

我有一些像这样的结构列表：

struct va_data
{
  public int item_id;
  public int type_id;
  public double item_value;
  public DateTime value_date;
}

我尝试按type_id对列表进行分组并获取value_date最大的项目然后按item_id分组，并仅在item_value最小的情况下获取项目

有我的语法

from x in dataList 
group x by x.type_id into grouped 
select grouped.Where(x => x.value_date == grouped.Max(y => y.value_date))
    .GroupBy(x => x.item_id) // and here i was stuck.


Example

var dataList = new []
{
   new va_data {item_id = 1, type_id = 1, item_value = 0, value_date = "2013.07.29"},
   new va_data {item_id = 1, type_id = 1, item_value = 1, value_date = "2013.07.30"},
   new va_data {item_id = 2, type_id = 1, item_value = 0, value_date = "2013.07.29"},
   new va_data {item_id = 2, type_id = 1, item_value = 1, value_date = "2013.07.29"},
   new va_data {item_id = 4, type_id = 2, item_value = 5, value_date = "2013.07.29"},
   new va_data {item_id = 4, type_id = 3, item_value = 9, value_date = "2013.07.30"},
};

The result must be 

var dataListResult = new []
{
   new va_data {item_id = 1, type_id = 1, item_value = 1, value_date = "2013.07.30"},
   new va_data {item_id = 2, type_id = 1, item_value = 0, value_date = "2013.07.29"},
   new va_data {item_id = 4, type_id = 2, item_value = 5, value_date = "2013.07.29"},
}

Answer 1

给出以下课程

class va_data
{
  public int item_id;
  public int type_id;
  public double item_value;
  public DateTime value_date;
}

和您的示例数据，您可以使用如下查询：

from data in dataList
group data by new {data.item_id, data.type_id} into g
let max_value_date = g.Max(x => x.value_date)
from i in g.Where(x => x.value_date == max_value_date)
group i by i.item_id into g2
let min_item_value = g2.Min(x => x.item_value)
from x in g2
where x.item_value == min_item_value
select x;

获得以下结果：

Answer 2

只需将您的查询分成两部分 - 获取每种类型的最新内容，然后获取每个项目的最小值：

var latestOfEachType = 
             from d in dataList
             group d by d.type_id into typeGroup
             select typeGroup.OrderByDescending(x => x.value_date).First();

var result = from d in latestOfEachType
             group d by d.item_id into itemGroup
             select itemGroup.OrderBy(x => x.item_value).First();

此查询将作为单个查询执行。但在这种情况下，它看起来更具可读性。 don't use mutable structs!也是{{3}}。改为使用类。

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编辑：因此你有几个项目的最大日期，然后查询需要两个小调整 - 选择日期最大的所有项目，并使用SelectMany迭代它们：

var latestOfEachType =
             from d in dataList
             group d by d.type_id into typeGroup
             let maxDate = typeGroup.Max(x => x.value_date)
             select typeGroup.Where(x => x.value_date == maxDate);

var result = from d in latestOfEachType.SelectMany(g => g)
             group d by d.item_id into itemGroup
             select itemGroup.OrderBy(x => x.item_value).First();