C# Skips first row but want it to skip last row instead

Question

Not sure if this is the best way but here's my code so far. Currently, it keeps the first duplicate and deletes the others from the table. I want it to keep the last row with the largest OrderId number and delete the rest. I've tried Take instead of Skip but can't seem to get it working properly.

var duplicateRow = (from o in db.Orders
                    group o by new { o.CustomerId } into results
                    select results.Skip(1)
                    ).SelectMany(a => a);

db.Orders.DeleteAllOnSubmit(duplicateRow);
db.SubmitChanges();

Answer 1

Since you don't use OrderBy the result is arbitrary

I want it to keep the last row with the largest 'OrderId' number

Then use:

var duplicateRows =  db.Orders
    .GroupBy(x => x.CustomerId)
    .SelectMany(g => g.OrderByDescending(o => OrderId).Skip(1));

Answer 2

You can use Reverse():

using System;
using System.Linq;

public class Program
{
    public class O
    {
        public int CustomerId;
        public int OrderId;
        public O(int c,int o) 
        {
            CustomerId = c; 
            OrderId = o;
        }

        public override string ToString() 
        {
            return string.Format("(C_ID: {0}, O_ID:{1})",CustomerId, OrderId);
        }
    }


    public static void Main()
    {
        var data = new O[]{ new O(1,1),new O(2,1),new O(3,1),new O(1,2)};

        var duplicateRow = (from o in data
                            group o by new { o.CustomerId } into results
                            select results.Reverse().Take(1) // reverse and then take 1
                           ).SelectMany(a => a);

        foreach( var o in duplicateRow)
                Console.WriteLine(o);

        Console.ReadLine();
    }
}

Output:

(C_ID: 1, O_ID:2)
(C_ID: 2, O_ID:1)
(C_ID: 3, O_ID:1)

This works due to stableness - if your input have later OrderID earlier you need to OrderBy as well.