一个简单的问题。关于删除一个记录的问题。

procedure TF_Select.BitBtn1Click(Sender: TObject);
begin
    begin
       if RadioButton1.Checked then                       //选择课程
        begin
              if (length(edit1.Text)<>4) or (Length(Edit2.Text)<>3) then
              begin
                messagedlg('请输入正确的课程号和教师号',mtError,[mbok],0);
                exit;
              end;
              with DataModule1.ADOQ_Cselect do
              begin
                sql.Clear ;
                sql.Add('select cno,tno from cselect where cno=:cno AND tno=:tno');
                parameters.ParamByName('cno').Value :=edit1.Text ;
                parameters.ParamByName('tno').Value :=edit2.Text ;
                open;
                if recordcount <>0 then
                begin
                  messagedlg('你已经选了这门课程',mterror,[mbok],0);
                  exit;
                end;
                close;
              end;              begin
                  Edit1.Text:='';
                  Edit2.Text:='';
              end;              with DataModule1 do
                 begin
                    ADOQ_Cselect.Close;
                    ADOQ_Cselect.SQL.Clear;
                    ADOQ_Cselect.SQL.Add('INSERT INTO cselect(sno,cno,tno) VALUES("'+username+'",'+edit1.text+','+edit2.text+')');
                    ADOQ_Cselect.ExecSQL;
                 end;
                 ShowMessage('选课成功');
          end;
     if RadioButton2.Checked then                        //删除选课记录
       begin
          with DataModule1 do
          begin
           ADOQ_Cselect.Close;
           ADOQ_Cselect.SQL.Clear;
           ADOQ_Cselect.SQL.Add('DELETE FROM cselect WHERE cno="'+Edit1.Text+'" AND tno="'+Edit2.Text+'"');
           ADOQ_Cselect.Open;
          end;
       end;
    end;
end;
procedure TF_Select.btn1Click(Sender: TObject);              //刷新选课信息
begin
      with DataModule1 do
      begin
           ADOQ_Cselect.Close;
           ADOQ_Cselect.SQL.Clear;
           ADOQ_Cselect.SQL.ADD('SELECT cselect.cno AS 课程号,cinfo.cname AS 课程名称,cselect.tno AS 教师号,cinfo.tname AS 教师名,cinfo.credit AS 学分,cinfo.cplace AS 上课地点 ,cinfo.ctime AS 上课时间,cinfo.limitdept AS 限选院系 FROM cinfo,cselect WHERE ');
           ADOQ_Cselect.SQL.Add('cinfo.cno=cselect.cno AND cinfo.tno=cselect.tno AND cselect.sno="'+username+'"');
           ADOQ_Cselect.Open;
      end;
end;end.
在删除一个选课记录的时候报告错误说“不支持单一执行返回多个记录集”。为什么啊？