mdb数据连接、查询、填充数据源

string filename=System.Windows.Forms.Application.StartupPath+"\\db.mdb";
            string strConn = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" +filename+ "";
            OleDbConnection conn = new OleDbConnection(strConn);
            string sql = "select 单位,姓名,工号 from t1";
            //string sql = "select 单位,姓名,count(*) from t1 group by 单位,姓名";
            DataSet ds = new DataSet();
            conn.Open();
            OleDbDataAdapter Command = new OleDbDataAdapter(sql, strConn);
            Command.Fill(ds);
            conn.Close();
            conn.Dispose();            
            this.dataGridView1.DataSource = ds.Tables[0].DefaultView;