Can someone say if I my E-R diagram is wrong?

Question

drop database if exists movie_rentals;

create database movie_rentals;
use movie_rentals;

create table movie (
  movie_id int,
  title varchar(255),
  release_year year,
  length int,
  rating enum('G','PG','PG-13','R','NC-17'),
  category varchar(255), 
  primary key(movie_id)
);

create table actor (
  actor_id int,
  first_name varchar(255),
  last_name varchar(255),
  primary key(actor_id)
);

create table movie_actor (
  movie_id int,
  actor_id int,
  primary key(movie_id, actor_id),
  foreign key(movie_id) references movie(movie_id),
  foreign key(actor_id) references actor(actor_id)
);

create table customer (
  customer_id int,
  first_name varchar(255),
  last_name varchar(255),
  address varchar(255),
  postal_code varchar(255),
  district varchar(255),
  city varchar(255),
  country varchar(255),
  primary key(customer_id)
);

create table rental (
  rental_id int,
  customer_id int,
  movie_id int,
  rental_date datetime,
  amount decimal(5,2),
  primary key(rental_id),
  foreign key(customer_id) references customer(customer_id),
  foreign key(movie_id) references movie(movie_id)
);

My proposal:

I am not sure about the double line between movie and movie_actor. I tried to do some tests on MySQL to see if I could insert a movie without an actor on movie_actor table, but I couldn´t.

Answer 1

The columns you choose to be primary key won't be allowed to be NULL. So in this case movie_actor require that both movie_id and actor_id needs to be filled.