新手指针 rs.next

Question

我只想看到产品用户正在寻找它们，但是当第二个 if 被执行时，它将推送（指针或任何存在的东西）到下一个 ID（我拥有唯一的 ID，所以它会推送到无处）和结果为空。我希望你能理解我的问题:)。

    if (stmt.execute(
                        "SELECT * FROM products where ID=" + removeName)) {
                    rs = stmt.getResultSet();
    if (!rs.next()) {
                       m = "ID not found.";
                        return m;
                   }

Answer 1

首先，您的方法容易受到 SQL 注入的攻击。请前往 PreparedStatement。
Look at this simple example for using PreparedStatement

你应该这样做：

ResultSet rs = stmt.executeQuery("SELECT * FROM products where ID=" + removeName);
if (!rs.next()) {
      m = "ID not found.";
      return m;
}

Answer 2

在你的情况下，你可以去PreparedStatement 避免SQL-Injection 问题。

  PreparedStatement prodsQuery= con.prepareStatement("SELECT * FROM products where ID=?");
  prodsQuery.setInt(1,removeName);
  ResultSet rs = prodsQuery.executeQuery();
  if(!rs.next())
  {
        m = "ID not found.";
        return m;
   }

Answer 3

问题是您正在阅读第一个结果以了解是否至少有一个结果，然后尝试使用下一个结果并错过第一个结果（改编自您的问题描述）。我解释了这是如何工作的here。

此问题的一个可能解决方案是假设查询执行没有问题并且您有结果，然后检索数据（或数据的List）并作为最后一步验证数据是否不为空或List 的数据不为空。

代码改编自 Naveen's answer 以显示建议的解决方案

PreparedStatement prodsQuery =
    con.prepareStatement("SELECT * FROM products where ID=?");
prodsQuery.setInt(1,removeName);
ResultSet rs = prodsQuery.executeQuery();

1. 假设只有一个结果可以得到：

   //also assuming you will set the results in a Data class (yes, this can be replaced)
   Data data = null;
   if (rs.next()) {
       //logic to retrieve data...
       data = new Data();
       data.setSomething(rs.get(1));
       //more and more code to fill the data...
   
       //because it looks that you need it as String (wonder why you return a String as well)
       return data.toString();
   }
   //note: I use an else statement to check if indeed there were no results at all
   //else statement added using a line separator for code explanation purposes
   else {
       m = "ID not found.";
       return m;
   }
   

2. 假设有一个要获取的结果列表：

   //also assuming you will set the results in a Data class (yes, this can be replaced)
   List<Data> dataList = new ArrayList<Data>();
   while (rs.next()) {
       //logic to retrieve data...
       Data data = new Data();
       data.setSomething(rs.get(1));
       //more and more code to fill the data...
   
       //because it looks that you need it as String (wonder why you return a String as well)
       dataList.add(data);
   }
   //in this case, there's no validation in order to know if there's any result
   //the validation must be in the client of this class and method checking if
   //the result list is empty using if(!List#isEmpty) { some logic... }
   return dataList;