这是我的程序代码:(绝对正确我以测试了),我想判断我"正在插入的新的username"是否
存在于库savemoney中,如果存在,就弹出对话框,说username以存在了,请重新输入,
//触发事件对"新用户的提交:"
          void jButton1_actionPerformed(ActionEvent e) {
                     oEreport = new ereport();//为取出id作准备
                     try{
                        oEreport.connectToDB();//取出记录数
                      }
                      catch(Exception e1)
                      {
                        System.out.println(e1);
                      }
                     strName = this.jTextFieldnewname.getText();
                     floMoney = Float.valueOf(jTextFieldmoney.getText());
                     strType = this.jComboBoxtype.getSelectedItem().toString();
                     if(strType =="用户名") {
                            String U="U";
                            strType = U;
                      }
                      if(  strType== "组名"){
                            String G="G";
                            strType = G;
                      }
                      ResultSet resDate = null;//为取出id作准备
                 try{

                        String   strSql = "select max("id") as maxnum from Savemoney";
                        resDate = oEreport.executeQuery(strSql);
                        if(resDate.next()){
                          iCount = resDate.getInt("maxnum");
                          iCount=iCount+1;
                        }
                      }
catch(Exception e1)
           {
    System.out.println(e1);
      }
                 try{
                     //插入记录
                     strSql = "Insert into Savemoney "+
                                     "Values ("+iCount+",'"+strName+"','"+strType+"',getdate(),"+floMoney +")";
                     // System.out.println(strSql);
                     int iOK=oEreport.executeUpdate(strSql);
                                if(iOK==1){
                                JOptionPane.showMessageDialog(null, "更新成功!", "更新成功!", JOptionPane.ERROR_MESSAGE);
                                }
                                else{
                                  JOptionPane.showMessageDialog(null, "更新失败!", "更新失败!", JOptionPane.ERROR_MESSAGE);
                                }
                     }
                   catch(Exception e1){
                     System.out.println(e1);
                     }
                 }

select count(username) from Savemoney where username = ".."
如返回纪录数>0 即纪录存在

实际上
ResultSet resDate = null;
String  strSql = "select * from Savemoney where username = '" + username + "'";
resDate = oEreport.executeQuery(strSql);
if(resDate!=null)
{
说明有纪录
}

一个简单的办法是设置username为primary key。在插入失败后不处理（插入不成功，就是key重复。
另一个办法是，每次插入前检查：）