LINQ：如何通过group by子句获取最大ID？

Question

I am looking for a way in LINQ to get a max Id record by using 'Group By' clause

Consider the following Sample data

  Table: ProcessAud

  ProcessSeq     ProjectSeq   ProjectValue    Active
      11              1           50000          Y
      12              1           10000          Y
      13              2           70000          Y 
      14              2           90000          Y

In which I want to get two records as a list such that is second and fourth
records (ie) ProcessSeq 12 and 14. And I tried it like following

var ProcessAudList = ProcessAudService.FilterBy(x => x.Active == "Y"    
  ).GroupBy(x => x.ProjectSeq).Max().ToList();

It is not working properly, So how to do it in LINQ. Please anybody help.

Answer 1

You want to pick top record from each group.

var ProcessAudList = ProcessAudService.Where(x => x.Active == "Y")
.GroupBy(x => x.ProjectSeq, (key,g)=>g.OrderByDescending(e=>e.ProjectValue).First());

Check demo code

Answer 2

When you use GroupBy extension, method will return you IGrouping instance and you should query IGrouping instance like below;

var ProcessAudList = collection.Where(x => x.Active == "Y").GroupBy(x => x.ProjectSeq).Select(x => x.OrderByDescending(a => a.ProcessSeq).FirstOrDefault()).ToList();

Hope this helps

Answer 3

You're most of the way there, but Max is the wrong term to use.

Each IGrouping is an IEnumerable (or IQueryable ) sequence of its own, so you can use OrderBy and First clauses to get the answer you need:

var ProcessAudList = ProcessAudService
    .FilterBy(x => x.Active == "Y")
    .GroupBy(x => x.ProjectSeq)
    .Select(grp => grp.OrderByDescending(x => x.ProcessSeq).First())
    .ToList();

The Select clause will process each of the groups, order the groups descending by ProcessSeq and select the first one. For the data you provided this will select the rows with ProcessSeq equal to 12 and 14.

Answer 4

With this code you can get all max id in foreach

var res = from pa in ProcessAud
      group Cm by pa.ProjectSeq into Cm1
      select new
      {
           _max = Cm1.Max(x => x.ProcessSeq)
      };
foreach (var item in res)
{
    //item._max have biggest id in group
}