LINQ 如何返回最后日期以及第一次和最后一次计数之间的差异

问题描述

我有一个包含以下列和示例数据的表格:

I have a table with the following column and sample data:

acteename   updated_at              count (count is not sorted in db)

dev-52      2/7/2020 5:56:43 PM     1
dev-52      2/7/2020 5:56:52 PM     2
dev-52      2/7/2020 5:57:00 PM     3
dev-52      2/7/2020 5:57:49 PM     4
dev-52      2/7/2020 5:59:19 PM     5
dev-52      2/7/2020 6:01:51 PM     6
dev-52      2/7/2020 6:06:21 PM     7
dev-52      2/7/2020 6:14:51 PM     8
dev-52      2/7/2020 6:31:23 PM     9
dev-52      2/7/2020 6:47:54 PM     10
dev-52      2/7/2020 7:04:26 PM     11
dev-52      2/7/2020 7:20:58 PM     12

我想返回按 updated_at 和最后一个 updated_at 排序的 first 和 last 计数之间的差异其中 acteename 等于我从不同表中获得的某个名称

I want to return diff between first and last count sorted by updated_at and last updated_at where acteename is equal to some name that I get from different table

IQueryable<Events> list = _DBcontext.Events.GroupBy(p => p.ActeeName)
                            .Select(g => new
                            {
                                acteename = g.Key,
                                lastupdated = g.OrderByDescending(p => p.UpdatedAt).First(),
                                firstcount = g.OrderBy(p => p.UpdatedAt).First().Count,
                                lastcount = g.OrderByDescending(p => p.UpdatedAt).First().Count
                            })
                            .Select(x => new { x.acteename,x.lastupdated, diff = x.lastcount - x.firstcount })
                            .Where(a => a.acteename == item.AppName)
                            .ToList();

出现以下错误:

不能隐式转换类型'System.Collections.Generic.List<>' to'System.Linq.IQueryable'.一个明确的转换存在(您是否缺少演员?)

预期输出:

dev-52      2/7/2020 7:20:58 PM     11

有点坚持不知道 LINQ 查询是否正确，任何帮助都会有用:)

kinda stuck on this not sure if the LINQ query is correct or not, any help will be useful :)

更新:

我运行的确切查询:

var list = from p in _DBcontext.AppProcessCrashEvents
                                   where p.Actee == item.AppGuid
                                   group p by p.Actee
            into gList
                                   select new
                                   {
                                       acteenamex = gList.Key,
                                       lastUpdated_at = gList.OrderBy(x => x.UpdatedAt).Last().UpdatedAt,
                                       diff = gList.OrderBy(x => x.UpdatedAt).Last().CrashCount -
                                                             gList.OrderBy(x => x.UpdatedAt).First().CrashCount
                                   };

推荐答案

一个人可以在一开始就应用过滤器本身.使用 group by 和 into 然后最后选择你需要的.由于它创建了一个匿名对象，因此最好以var"开头.

One can apply the filter in very beginning itself. Use group by with into and then finally select what you need. Since its creating a anonymous object hence it would be better to start with 'var`.

获取内存中的数据，然后应用group by 以避免在与数据库端分组映射时出现任何问题.

Edited: Fetch the data in memory and then apply group by to avoid any problem in mapping with database side grouping.

var list =  (from p in _DBcontext.Events
            where p.acteename == item.AppName select p).ToList();

var fList = (from p in list 
            group p by p.acteename
            into gList
            select new
            {
               acteenamex = gList.Key,
               lastUpdated_at = gList.OrderBy(x => x.updated_at).Last().updated_at,
               diff = gList.OrderBy(x => x.updated_at).Last().count - 
                                      gList.OrderBy(x => x.updated_at).First().count
            }).ToList();

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