截取ds需要的列（ 截取DataTable的指定列）

 public static DataTable GetColumns(DataTable dt, string name)             //截取datatable里面的name列作为新datatable
        {
            DataTable tmp = new DataTable();   定义一个新的表
            tmp.Columns.Add(name);

            int rowsnum = dt.Columns[name].Table.Rows.Count;           

             for (int i = 0; i < rowsnum; i++)
            {
                tmp.Rows.Add();
                try
                {
                    tmp.Rows[i][name] = dt.Rows[i][name];
                }
                catch { }
            }
            return tmp.Copy();
        }