Question

我正在开发一个场景，我必须在JDBC中执行SQL查询，以查找有关在特定主题中注册的学生的信息。运行此查询时，应一次显示3条记录。然后提示用户按Enter返回主菜单或按B继续浏览结果或输入StudentID以提取前一学期的结果。在下面的代码中，显示3个结果后，当用户按Enter或B时，我的代码可以正常工作。但是，在输入StudentID并运行子查询后，代码会自动显示主菜单。我希望它返回并继续显示剩余的结果，就像选项B一样浏览。

以下是代码段：

// Below is the SQL query that will be executed
// This snippet is part of loop where all different subjects are entered into an array and then user selects one subject
String displayStudent = "select * from students where subject = '" + subjectArray[choice - 1] + "'";
ResultSet displayThisList = stmt.executeQuery(displayStudent);
while (displayThisList.next()) {
    System.out.println("Fname: " + displayThisList.getString(2));
    System.out.println("Lname: " + displayThisList.getString(3));
    System.out.println("StudentID: " + displayThisList.getString(1));
    System.out.println("Grade: " + displayThisList.getString(4));
    System.out.println("Subject: " + displayThisList.getString(5));
    System.out.println();
    subCount++;
    if ((subCount % 3 == 0)) {
        String response = readEntry("Enter StudentID to view last semester results or\nB Enter to browse or \nENTER to go back:");
        System.out.println();
        if (response.equals("")) {
            // Back to member menu
            break;
        } else if (response.equals("B")) {
            // Continue to browse
            continue;
        } else {
            // Check if the StudentID is valid or not
            int checkID = 0;
            String queryID = "select * from students where StudentID='" + response + "'";
            ResultSet setID = stmt.executeQuery(queryID);
            while (setID.next()) {
                String FName = setID.getString(2);
                String LName = setID.getString(3);
                String Grade = setID.getString(4);
                String Subject = setID.getString(5);
                checkID++;
                System.out.println("Correct ID was entered");
                //Code to execute query based on above info from a different table
            }
            if (checkID == 0) {
                System.out.println();
                System.out.println("Invalid ID. Please enter again.");
                System.out.println();
            }
        }
    }
}

Answer 1

首先，对于不同的查询，您不应使用相同的Statement对象如果您可以互换使用相应的ResultSet。为“外部”查询创建一个语句，为子查询创建另一个语句。

其次，在while - 循环（setId.next()）中，您可能会从错误的ResultSet（应setID.getString()，而不是setISBN.getString()）读取数据。

此外，如果基于用户输入生成SQL查询，您肯定应该使用PreparedStatement。当有人输入StudentId时，请考虑将会发生什么：

'; delete from students; select * from students where StudentID='foobar

执行子查询后无法返回JDBC SQL查询结果

1 个答案: