Ключи в таблицах в БД SQL

create database movies
go
use movies
go
create table movies
(
    MovieID int primary key identity,
    DirectorID int not null,
    Title nvarchar(100) not null,
    ReleaseYear int not null,
    Rating int not null check (Rating >=0 and Rating <= 100),
    Plot ntext,
    MovieLength int not null check(MovieLength > 0)
);
go
create table MovieGenres
(
    MovieID int primary key identity,
    GenreID int primary key identity
);
go
create table Genres
(
    GenreID int primary key identity,
    GenreName nvarchar(100) not null
);
go
create table MovieActor
(
    MovieID int primary key identity,
    ActorID int primary key identity
);
go
create table Actors
(
    ActorID int primary key identity,
    FirstName nvarchar(100) not null,
    LastName nvarchar(100) not null,
    Nationality nvarchar(100) not null,
    Birth int not null
);
go
create table Directors
(
    DirectorID int primary key identity,
    FirstName nvarchar(100) not null,
    LastName nvarchar(100) not null,
    Nationality nvarchar(100) not null,
    Birth int not null
);
go