如何在 LINQ 中编写查询和使用 distinct 和 order by？

Question

Can you please help me how to write this query in LINQ syntax

select distinct machine_id, sample_id from results where custid = 18 order by sample_id

I tried this but not working like the query it shows only machine_id in sample no 1 but I need to select distinct machine_id for each sample

 public ActionResult Indexs() { int UserId = Convert.ToInt32(Session["UserID"]); var samples = _context.RESULTS.GroupBy(mach => mach.machine_id).Select( x=> x.FirstOrDefault()).Where(x=> x.custid == UserId).ToList(); return View(samples); }

I cannot find DistinctBy<> only Distinct available I tried the solutions in this question but its not cover my case

https://stackoverflow.com/questions/14321013/distinct-in-linq-based-on-only-one-field-of-the-table

please I need your help and thank you in advance

UPDATE:

Still the solution not working as expected and not show all data this is the code I use:

 var samples = _context.RESULTS.GroupBy(mach => new { mach.machine_id, mach.sample_id }).Select(x=>x.FirstOrDefault()).OrderBy(n=> new { n.sample_id,n.program_id }).Where(x => x.custid == UserId).ToList();

This is example what I have in RESULTS table and what I need to select:

 user sample no 1 machine_id 1 1 4 1 1 2026 1 1 2030 1 1 2046 1 1 2053 1 1 2058 1 1 2061 1 1 2080 1 1 2081 1 1 2083 1 1 2084

but with this command it shows only 4 machines:

2080

2081

2083

2084

and I think groupby and firstordefault cause this please how to solve it and select distinct machine_id from each sample like the SELECT statement why its very difficult to make simple query in LINQ?

this is the VIEW code, I dont know if it will be useful:

 @model IEnumerable<warehouse.Models.RESULT> @{ ViewBag.Title = "Indexs"; Layout = "~/Views/Shared/_LayoutDashboard.cshtml"; } <img style="margin-left:250px;" src="~/images/weblogo.png" /> <p style="margin-left:40px;"> <h3 style="margin-left:100px; font-family:Andalus;text-underline-position:below"> @Html.Label("Hospital Name:") @Html.DisplayFor(model => Model.FirstOrDefault().sys_users.user_full_name) </h3> <table class="table table-bordered"> <tr style="background-color:hotpink"> <th> @Html.DisplayNameFor(model => model.Program.name) </th> <th> @Html.DisplayNameFor(model => model.sample.name) </th> <th> @Html.DisplayNameFor(model => model.Machine.Machine_name) </th> <th></th> </tr> @foreach (var item in Model) { <tr> <td> @Html.DisplayFor(modelItem => item.Program.name) </td> <td> @Html.DisplayFor(modelItem => item.sample.name) </td> <td> @Html.DisplayFor(modelItem => item.Machine.Machine_name) </td> <td> @Html.ActionLink("Enter", "Edit", new { id = item.sample_id, programId = item.program_id, custId = item.custid, machineId = item.machine_id }, new { @class = "btn btn-primary" }) | @Html.ActionLink("Report", "Details", new { id = item.sample_id, programId = item.program_id, custId = item.custid }, new { @class = "btn btn-danger" }) | @Html.ActionLink("Statistics", "Edit", "Statistics", new { programId = item.program_id, hospitalNo = item.custid }, new { @class = "btn btn-info" }) </td> </tr> } </table>

Answer 1

here is an example using distinctBy

Obj obj0 = new() { Id = 1, SimpleId = 1, Name = "name1" }; Obj obj1 = new() { Id = 1, SimpleId = 1, Name = "name2" }; Obj obj2 = new() { Id = 1, SimpleId = 1, Name = "name1" }; Obj obj3 = new() { Id = 1, SimpleId = 1, Name = "name2" }; Obj obj4 = new() { Id = 1, SimpleId = 2, Name = "name4" }; Obj obj5 = new() { Id = 1, SimpleId = 2, Name = "name2" }; Obj obj6 = new() { Id = 1, SimpleId = 2, Name = "name3" }; List<Obj> list = new() { obj0,obj1,obj2, obj3, obj4, obj5, obj6}; var result = list.OrderByDescending(o => o.Id).DistinctBy(p => new{p.SimpleId, p.Name}).OrderBy(o => o.Id); Console.WriteLine( "liste count " + list.Count); int n = 0; foreach (Obj obj in result) { Console.WriteLine(++ n + " " +obj); } record Obj { public int Id { get; set; } public int SimpleId { get; set;} public string Name { get; set; } }

result display

liste count 7 1 Obj { Id = 1, SimpleId = 1, Name = name1 } 2 Obj { Id = 1, SimpleId = 1, Name = name2 } 3 Obj { Id = 1, SimpleId = 2, Name = name4 } 4 Obj { Id = 1, SimpleId = 2, Name = name2 } 5 Obj { Id = 1, SimpleId = 2, Name = name3 }

you see all duplicates are deleted

Answer 2

You may have typos in the query or using wrong properties. Below is getting correct results. I split the orderby into two pieces.

 using System; using System.Linq; using System.Text; using System.Collections; using System.Collections.Generic; using System.Data; namespace ConsoleApp2 { class Program { static void Main(string[] args) { Context _context = new Context() { RESULTS = new List<RESULTS>() { new RESULTS() { user = 1, sample_id = 1, machine_id = 4 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2026 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2024 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2038 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2046 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2053 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2058 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2061 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2080 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2081 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2083 }, new RESULTS() { user = 1, sample_id = 1, machine_id = 2684 } } }; int UserId = 1; var samples = _context.RESULTS.GroupBy(mach => new { mach.machine_id, mach.sample_id }).Select(x => x.FirstOrDefault()).OrderBy(n => n.sample_id ).ThenBy(x => x.machine_id).Where(x => x.user == UserId).ToList(); } } public class Context { public List<RESULTS> RESULTS { get; set; } } public class RESULTS { public int user { get; set; } public int sample_id { get; set; } public int machine_id { get; set; } } }

Answer 3

I find that your question post (as it currently stands) leaves some parts open to interpretation.

My understanding is that the displayed RESULTS selection

user_id sample_id machine_id 1 1 4 1 1 2026 1 1 2030 1 1 2046 1 1 2053 1 1 2058 1 1 2061 1 1 2080 1 1 2081 1 1 2083 1 1 2084

is in fact the selection you get when executing

select distinct user_id, sample_id, machine_id
from results
where user_id = 1
order by sample_id

, whereas the RESULTS table in your database may contain more entries than the 11 that are displayed.

Is that a correct assumption?

(If no, please inform me and I will delete this answer.)

If so, I believe that there may be entries in the RESULTS table where { sample_id, machine_id } are equal, but where user_id differ.

As an example, RESULTS may contain two entries where sample_id == 1 and machine_id == 4 :

 user_id sample_id machine_id ------------------------------- 2 1 4 1 1 4......... // other entries

When grouping these two entries by { sample_id, machine_id } , you will get one grouping containing two elements:

 Key: { 1, 4 } Elements: { 2, 1, 4 }, { 1, 1, 4 }

Selecting the first element in that grouping will give you

{ 2, 1, 4 }

, simply because { 2, 1, 4 } appeared before { 1, 1, 4 } in the table. We basically disregarded the database table entry { 1, 1, 4 } before even getting to the .Where() statement.

Now, by using .Where() to get only the elements that are associated with user_id == 1 , the element { 2, 1, 4 } will be excluded , seeing as user_id == 2 .

---

My suggestion is therefore to change the order of method calls, so that .Where() is used prior to the grouping and selection of the first group element. A minimal example:

 var filteredResults = results.Where(r => r.user_id == 1) // filtering before grouping.GroupBy(r => new { r.machine_id, r.sample_id }).Select(r => r.First());

When doing this, the grouping will only be based on entries where userId == 1 , and therefore, no entry associated with user_id == 1 will be "lost" in the stage of selecting the first element in each grouping.

This fiddle may help in displaying the difference regarding the order of method calls.