//先得到两个DataTable
DataTable DataTable1 = SqlHelper.GetDataTable("select convert(char(5),id) As 'id' ,num from testSum");
DataTable DataTable2 = SqlHelper.GetDataTable("select '统计',Sum(num) from testSum");
//复制新建一个新的DataTable
DataTable newDataTable = DataTable1.Clone();

//将DataTable1的数据加入到新的DataTable中 
object[] obj = new object[newDataTable.Columns.Count];
for (int i = 0; i < DataTable1.Rows.Count; i++)
{
    DataTable1.Rows[i].ItemArray.CopyTo(obj, 0);
    newDataTable.Rows.Add(obj);
}
//将DataTable2的数据加入到新的DataTable中
for (int i = 0; i < DataTable2.Rows.Count; i++)
{
    DataTable2.Rows[i].ItemArray.CopyTo(obj, 0);
    newDataTable.Rows.Add(obj);
}
//显示到GridView中
GridView1.DataSource = newDataTable;
GridView1.DataBind();

下边的话是我在项目中实用到的例子：

        DataTable DtRegister = GetRegister("select * from registration",mysqlConnString).Tables[0];
        DataTable DtSource = bll.GetList3(strWhere).Tables[0];
        DataTable result = DtSource.Clone();
        int i = 0;
        object[] obj=new object[DtSource.Columns.Count];
        foreach (DataRow dataRow1 in DtSource.Rows)
        { 
            foreach (DataRow dataRow2 in DtRegister.Rows)
            {
                if(dataRow1["HKID"].ToString().Equals(dataRow2["regist_number"].ToString()))
                {
                   DtSource.Rows[i].ItemArray.CopyTo(obj,0);
                    result.Rows.Add(obj);
                    i++; 
                }
            }
        }