Question

I have a DataTable with 8 fields. I want to return another DataTable that groups the first 7 fields and has the max value of the 8th. All fields are strings, the 8th is a number stored in a string field, e.g. “24”.

Code that I’ve tried so far:

public DataTable highestVersion(DataTable dt)
        {
            DataTable dtResult=dt.Clone();

            var query=from dtRow in dt.AsEnumerable()
                      group dtRow by new
                      {
                          b_r = dtRow["r"],
                          b_1 = dtRow["b_1"],
                          b_2 = dtRow["b_2"],
                          p_r = dtRow["p_r"],
                          p_1 = dtRow["p_1"],
                          p_2 = dtRow["p_2"],
                          p_f = dtRow["p_f"]
                      }
                          into maxVersion
                          select maxVersion.OrderByDescending(a => a["p_v"]).First();

            foreach (var result in query)
            {
                 dtResult.ImportRow(result);
            }
            return dtResult;
        }

My expectation is that the group dtRow by new{} should group by the first 7 and get only the highest element p_v of each grouping by calling OrderByDescending().First(). But this doesn’t seem to actually do anything; all of the input rows are being returned.

EDIT: I just realized what the problem is. The values in p_f are different from one another, e.g.

datarow 1
--------
r: "abc"
b_1: "def"
b_2: "ghi"
p_r: "jkl"
p_1: "mno"
p_2: "pqr"
p_f: "stu_this"
p_v: "18"

datarow 2
--------
r: "abc"
b_1: "def"
b_2: "ghi"
p_r: "jkl"
p_1: "mno"
p_2: "pqr"
p_f: "stu_that"
p_v: "24"

In this case I would want to only return DataRow 2, because 24>18, and be able to retrieve the stu_that value.

Answer 1

Updated the answer in respond to comments:

var res = dt.AsEnumerable()
            .GroupBy(dtRow => new
            {
                b_r = dtRow["r"],
                b_1 = dtRow["b_1"],
                b_2 = dtRow["b_2"],
                p_r = dtRow["p_r"],
                p_1 = dtRow["p_1"],
                p_2 = dtRow["p_2"]
            })
            .Select(g => new
            {
                Group = g,
                Max = g.Max(r => r["p_v"])
            })
            .Select(g => new
            {
                Key = g.Group.Key,
                Max = g.Max,
                Values = g.Group
                          .Where(r => r["p_v"].Equals(g.Max))
                          .Select(r => r["p_f"])
            });

I guess it could be optimized to some extent, but at least this should do the job, if all the conditions are known now, of course.