using System.Collections.Generic;
using System.Data;
using System.Diagnostics;
using System.Linq;
DataTable dt = new DataTable();
dt.Columns.Add("entry_date");
dt.Columns.Add("employee_id");
dt.Columns.Add("employee_name");
dt.Columns.Add("details");
dt.Rows.Add(new string[] { "10/11/2018", "2", "PRINCE", "TYPING" });
dt.Rows.Add(new string[] { "10/11/2018", "1", "KING", "PRODUCTION" });
dt.Rows.Add(new string[] { "11/11/2018", "7", "RAVI", "ADMIN" });
dt.Rows.Add(new string[] { "11/11/2018", "3", "KANNAN", "SPORTS" });
dt.Rows.Add(new string[] { "11/11/2018", "2", "PRINCE", "HR" });
dt.Rows.Add(new string[] { "12/11/2018", "4", "KUMAR", "ACCOUNTS" });
dt.Rows.Add(new string[] { "13/11/2018", "7", "RAVI", "HR" });
dt.Rows.Add(new string[] { "13/11/2018", "5", "RAMAN", "PRODUCTION" });
IEnumerable<DataRow> result = dt.AsEnumerable()
.OrderBy(r => r.Field<string>("entry_date"))
.ThenBy(r => r.Field<string>("employee_id"))
.GroupBy(r => r.Field<string>("entry_date")).Select(g => g.First());
foreach (var dtrow in result)
{
Debug.Print(string.Join("\t", dtrow.ItemArray));
}