Linq，如何进行分组? [英] Linq, How to do groupBy?

问题描述

项目表:

编号，部门编号，年份，名称，级别

Id, DeptId, Year, Name, Level

Id = 1, DeptId = 1, Year = 2000, Name = "ABC", Level = 1
Id = 2, DeptId = 1, Year = 2001, Name = "ABC1", Level = 1
Id = 3, DeptId = 1, Year = 2002, Name = "ABC2", Level = 1

Id = 4, DeptId = 2, Year = 2000, Name = "ABC3", Level = 1
Id = 5, DeptId = 2, Year = 2002, Name = "ABC4", Level = 1

Id = 6, DeptId = 3, Year = 2000, Name = "ABC5", Level = 1
Id = 7, DeptId = 3, Year = 2001, Name = "ABC6", Level = 1
Id = 8, DeptId = 3, Year = 2002, Name = "ABC7", Level = 1

我有一个项目表.我需要为2001年的每个部门获取项目.对于DeptId = 2，它没有2001年的项目，我需要显示上一年的项目为2000.

I have a project table. I need to get the project for each dept for 2001. for DeptId = 2, it has no project for 2001, I need to show previous year project which is 2000.

我的linq应该返回以下结果.

my linq should return following result back.

Id = 2, DeptId = 1, Year = 2001, Name = "ABC1", Level = 1
Id = 4, DeptId = 2, Year = 2000, Name = "ABC3", Level = 1
Id = 7, DeptId = 3, Year = 2001, Name = "ABC6", Level = 1

我当时正在考虑使用groupby，对DeptId进行分组，但不确定如何编写此查询.

I was thinking about using groupby, group the DeptId, but not sure how to write this query.

更新:

这是我尝试过的.

var projects = project.Where(x => x.Year == 2001)

var projects = project.Where(x=>x.Year == 2001)

但是对于DeptId = 2，这将不会返回结果，这就是为什么我在考虑使用groupby的原因，但是却不知道如何编写它.

but this will not return result for DeptId = 2, that's why I am thinking about using groupby, but clueless how to write it.

推荐答案

var query= from p in context.Projects
           group p by p.DeptId into grp
           select grp.Where(x => x.Year <= 2001)
                     .OrderByDescending(x => x.Year)
                     .FirstOrDefault();

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