Use of Datatable Select method.

Suppose, we want to process individual user records so we can not write a query again and again and poll to database. It's time taking process.

So, to reduce it, what we will do,

We will fetch all records, Suppose, we have a table called 'employee master' containing huge data in it.
So, we will store in DataTable at once, meaning writing a below query

On the Page Load

Select * from Employee_Master order by emp_name;

ViewState["Employee_data"] = dt_employee;

//here dt_employee is your Datatable object which contains all Employee records.

Now, use Select method,

Suppose we want record of a particular Employee,then
in any events, where you want to populate particular Employee record, write

if(ViewState["Employee_data"]!=null)
{
  DataTable dt_emp = Ctype(ViewState["Employee_data"],DataTable);
  //Here Ctype is used for casting purpose, you can also use DirectCast instead on 
    Ctype

  If(dt_emp.rows.count>0)
  {
     //declare DataRow array as
     DataRow[] dr_employee = dt_emp.Select("emp_id='"& pass_employee_id &"'");

//Where emp_id is your column name and pass_employee_id is variable in which you will pass particular Employee-Id

Note: Select method returns always Array of DataRow

//Check if dr_employee is null or it's Length is 0 as

 if(dr_employee!=null)
      {
         if(dr_employee.Length>0)
         {
            //Now, take another DataTable as
            DataTable dt_new_employee = null;

            dt_new_employee = dr_employee.CopyToDataTable();

//Note: CopyToDataTable will give you datatable.
            
            dtl_view_employee_details.DataSource = dt_new_employee;
            dtl_view_employee_details.DataBind(); 
         }
      }
  }
}

So, here you learn Select method, it's useful for filtering/search criteria.
for this, you do not need to go to Database and fetch single row.