DateSet数据源的合并

try
        {
            DataSet dd = new DataSet();
            for (int i = 2004; i < 2007; i++)
            {
                OracleConnection conn = new OracleConnection();
                conn.ConnectionString = "server=localhost;uid=sa;pwd=sa;data source=haha;";
                conn.Open();
                string sql = " select sum(column1),TO_CHAR(avg(column2)), column3 from   table where column3 like '" + i + "%' group by column4";
                OracleCommand cmd = new OracleCommand();
                OracleDataAdapter da = new OracleDataAdapter(sql, conn);
                DataSet ds = new DataSet();
                da.Fill(ds);
                DataTable dt = ds.Tables[0];
                dd.Merge(dt,false,MissingSchemaAction.Add);
                conn.Close();
            }
            GridView1.DataSource = dd;
            GridView1.DataBind();
        }
        catch (Exception ex)
        {
            Response.Write(ex.Message);
        }