Create inner_join table or create new table MySQL

Question

I have two tables in my ClassSelector DB:

students

student_id | student_name | hometown

classes

class_id | classname | description

I am trying to write a program that will take a user-entered student_id and allow them to select a class from a list of class ID's. I want the application to write student_id, student_name, class_id, classname to a separate table. To do this, would it be best to create a new table(student_x_class) and use an INSERT statement:

INSERT INTO ClassSelector.student_x_class ClassSelector.students.student_id,ClassSelector.students.student_name, ClassSelector.classes.class_id,ClassSelector.classes.classname VALUES(W, X, Y, Z);

or create a new column in the classes table (student_x_class) and use this as a foreign key to the student_id column and use inner join.

Sorry if this is confusing, I'm new to MySQL. Any help would be appreciated!

ResultSet rs3 = myStmt3.executeQuery("SELECT * FROM ClassSelector.classes WHERE class_id = " + selectedClass);
    while(rs3.next()){
                            String innerJoin = (userEnterId + " has been added to " + rs3.getString("classname") + " " + rs3.getString("class_id"));
                            System.out.println(innerJoin);
                            String student_classJoin = "INSERT INTO students_x_classes" + "(student_name, class_id, classname)" + "VALUES (?, ?, ?)";
                            PreparedStatement pStmt = con.prepareStatement(student_classJoin);
                            pStmt.setString(1, userEnterId);
                            pStmt.setString(2, rs2.getString("class_id"));
                            pStmt.setString(3, rs2.getString("classname"));
                            pStmt.executeUpdate();
                            input.close();
                        }

Answer 1

Since student-classes is an M:N relationship (meaning 1 student can attend many classes and 1 class can have many students) I would suggest you create table student_x_class with columns (class_id, student_id) .

Then you can select all classes for a specific student with a simple join select

SELECT *
FROM students s
  INNER JOIN student_x_class sc ON sc.student_id = s.student_id
  INNER JOIN classes c ON c.class_id = sc.class_id
WHERE s.student_id = ?

When you want to INSERT rows into the student_x_class table, you don't need to join anything. Simply insert the class_id and student_id pair (no need for classname in that table), eg

String student_classJoin = "INSERT INTO student_x_class" + "(student_id, class_id)" + "VALUES (?, ?)";

And don't forget to adjust the binds, ie remove line pStmt.setString(3, rs2.getString("classname"));

Btw this is really basic stuff, try reading some SQL book/tutorial to get the whole concept.

Answer 2

That depends on the nature of the student-class relation. If a given class is only related to one student (let's imagine is its delegate or something like that) then a foreign key in classes table would be enough. If a class can be related to more than one student then you should think in creating a new table to model that relation. If you use this second approach, a good recommendation would be to include in the new table only the two ids.