Question

我有以下代码，我想确保user_entered_student_id等于现有的user_id。如何通过SQL语句将所有user_id与user_entered_student_id进行比较？

System.out.println("\nSign Up For a Class\n");
try {

    con = DriverManager.getConnection("jdbc:mysql://localhost:3306/ClassSelector?autoReconnect=true&useSSL=false", "root", "Volks91!");

    System.out.println("Enter Student ID: ");
    user_entered_student_id = sc.nextLine();
    String selectClassId = "SELECT class_id FROM ClassSelector.classes";
    myStmt = con.prepareStatement(selectClassId);
    rs = myStmt.executeQuery(selectClassId);
    while (rs.next()) {

        if (selectClassId.equals(user_entered_student_id)) {
            System.out.println("This is not a valid User ID");
        }

        else {
            String selectStudentFromIdQuery = ("SELECT student_name FROM ClassSelector.students WHERE student_id = " + user_entered_student_id);
            myStmt = con.prepareStatement(selectStudentFromIdQuery);
            rs = myStmt.executeQuery(selectStudentFromIdQuery);
            while (rs.next()) {
                userEnterIdAsName = rs.getString("student_name");
                System.out.println("Is " + userEnterIdAsName + " the correct student? (Y/N)");
                String confirm = sc.nextLine();

                if (confirm.equalsIgnoreCase("Y")) {
                    addClass();

                } else if (confirm.equalsIgnoreCase("N")) {
                    System.out.println("Oops, let start over");
                    return;
                }
            }
        }
    }
} catch ( java.sql.SQLException SQL ) {
    SQL.printStackTrace();
}

}

Answer 1

应用程序关闭的原因是，在您的else块中，查询不会为不存在的学生返回任何内容，因此while (rs.next())...

中没有任何内容循环

首先考虑运行检查作为结果集，如果数据为空，则next()返回false。另请注意SQL参数化的使用（防止sql注入）。此外，结果集可以滚动以在检查后准备好为现有学生循环时返回到第一位：

String selectStudentFromIdQuery = ("SELECT student_name FROM ClassSelector.students " +
                                   "WHERE student_id = ?");

myStmt = con.prepareStatement(selectStudentFromIdQuery,
                              ResultSet.TYPE_SCROLL_INSENSITIVE);
myStmt.setInt(1, user_entered_student_id);
rs = myStmt.executeQuery();

if (!rs.next()) {
    System.out.println("Not an actual student");
}
rs.beforeFirst();
while (rs.next()){    
    userEnterIdAsName = rs.getString("student_name");
    ...
}

将SQL select语句与用户输入的值进行比较

1 个答案: