public  static void letsGo(){//UPDATE
    con=null;
     try{
            con = DriverManager.getConnection("jdbc:hsqldb:hsql://localhost/","SA","");
            stmt = con.createStatement();


             ResultSet rs = stmt.executeQuery("select buchnr, autor from buecher, autoren where buecher.isbn = autoren.isbn");
             while(rs.next()){
                 int buchnr = rs.getInt(1);
                 String autor = rs.getString(2);

                 if(buchnr != altNr){

                     altNr = buchnr;
                     altAutor = autor;
                 }
                else{

                        System.out.println("UPDATE zweiautor set autor = '"+autor+", "+altAutor+"' where buchnr = "+altNr+" AND autor = '"+altAutor+"'");
                        stmt.executeUpdate("UPDATE zweiautor set autor = '"+autor+", "+altAutor+"' where buchnr = "+altNr+" AND autor = '"+altAutor+"'");
                        System.out.println("DELETE FROM zweiautor WHERE buchnr = "+buchnr+" AND autor = '"+autor+"'");
                        stmt.executeUpdate("DELETE FROM zweiautor WHERE buchnr = "+buchnr+" AND autor = '"+autor+"'");

                }
             }
         }
        catch(SQLException e){
            e.printStackTrace();
        }

     }

    public  static void letsRock(){//INSERT
    con=null;
     try{
            con = DriverManager.getConnection("jdbc:hsqldb:hsql://localhost/","SA","");
            stmt = con.createStatement();

             ResultSet rs = stmt.executeQuery("select buchnr, autor from buecher, autoren where buecher.isbn = autoren.isbn");
             while(rs.next()){

                 int buchnr = rs.getInt(1);
                 String autor = rs.getString(2);

                 stmt.executeUpdate("insert into zweiautor values("+buchnr+",'"+autor+"')");
                System.out.println("insert into zweiautor values("+buchnr+",'"+autor+"')");

             }
         }
        catch(SQLException e){
            e.printStackTrace();
        }

     }