我正在研究一个场景,我必须在 JDBC 中执行 SQL 查询来查找注册特定学科的学生的信息。运行此查询时,一次应显示 3 条记录。然后系统会提示用户按 Enter 返回主菜单,或按 B 继续浏览结果,或输入 StudentID 以提取上学期的结果。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,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 浏览一样。
下面是代码片段:
// 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();
}
}
}
}
最佳答案
首先,您不应该使用相同的 Statement
不同查询的对象如果您使用相应的ResultSet
可以互换。为“外部”查询创建一条语句,为子查询创建另一条语句。
其次,在 while
-loop ( setId.next()
) 您可能从错误的 ResultSet
读取数据(应该 setID.getString()
,而不是 setISBN.getString()
)。
此外,如果 SQL 查询是根据用户输入生成的,那么您绝对应该使用 PreparedStatement
反而。考虑一下当有人进入 StudentId
时会发生什么如:
'; delete from students; select * from students where StudentID='foobar
关于java - 执行子查询后无法返回 JDBC SQL 查询结果,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/15407209/