Java / MySQL中的if / else语句

Question

I am try to create a function where you enter a student_id saved in a MySQL database("student_id") and you will see the student name and what classes they are enrolled in. I am having trouble with the if/else statement, though. It works if the student_id entered is in the database, but if you enter an ID that isn't, it does not print out the error message I want; it instead just goes back to the menu.

System.out.println("\nStudent Enrollment\n");
      try {
          Scanner input = new Scanner(System.in);
          System.out.println("Enter Student ID to See What Classes they are enrolled in: ");
          String user_entered_student_id = input.nextLine();

          Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/ClassSelector?autoReconnect=true&useSSL=false", "", "");
          Statement myStmt = con.createStatement();

          ResultSet rs;
          rs = myStmt.executeQuery("SELECT student_id, student_name, class_id, classname  FROM ClassSelector.student_x_class WHERE student_id = " + user_entered_student_id);
          while(rs.next()){
              String studentInClass = (rs.getString("student_id") + "\t" + rs.getString("student_name") + "  " + rs.getString("class_id") + " " + rs.getString("classname"));
              if(!user_entered_student_id.equals("ClassSelector.students.student_id")){
                 System.out.println(studentInClass); 
              }
              else{
              System.out.println("This Student does not Exist!");
              }
          }   
      }

Answer 1

You have your logic in the while loop which will never be entered in no rows exist.

try

 boolean found = false;
 while(rs.next()){
              if(!user_entered_student_id.equals("ClassSelector.students.student_id")){
                 System.out.println(studentInClass); 
                 found = true;
                 break; // ??  Is this unique?
              }
  } 

  if (!found) {
      System.out.println("This Student does not Exist!");
      return false; // ???
  }
  return true; // ???

Of course if you are only look for one unique record then you can use if instead of while

Answer 2

The main reason is because if your connection will return an empty result set then the loop while(rs.next()){ } will not get executed.

rs.next returns false when there are no matching entries in the database.