Postgres 9.5

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create table genres
(
genre_id serial primary key,
genre_name varchar NOT NULL UNIQUE
);

create table movies
(
movie_id serial primary key,
movie_name varchar NOT NULL           
);

create table movie_genres
(
movie_id integer references movies NOT NULL,
genre_id integer references genres NOT NULL, 
PRIMARY KEY(movie_id, genre_id)
);
INSERT INTO genres
            (genre_name)
            VALUES ('g_A'),
                   ('g_B'),
                   ('g_C'),
                   ('g_D');
4 rows affected

INSERT INTO movies
            (movie_name)
            VALUES ('m_A'),
                   ('m_B'),
                   ('m_C'),
                   ('m_D');
4 rows affected

INSERT INTO movie_genres 
            (movie_id,
             genre_id)
SELECT rg.movie_id,
       rg.genre_id
       FROM movies m
            CROSS JOIN LATERAL ((SELECT g.genre_id,
                                        m.movie_id -- that's just here to force the optimizer to keep the join lateral
                                        FROM genres g
                                        ORDER BY random()
                                        LIMIT 1)
                                UNION
                                (SELECT g.genre_id,
                                        m.movie_id
                                        FROM genres g
                                        WHERE random() < 0.15)) rg;
7 rows affected

SELECT m.*,
       g.*
       FROM movies m
            INNER JOIN movie_genres mg
                       ON mg.movie_id = m.movie_id
            INNER JOIN genres g
                       ON g.genre_id = mg.genre_id;


movie_id
movie_name
genre_id
genre_name


1
m_A
2
g_B

2
m_B
4
g_D

2
m_B
2
g_B

3
m_C
2
g_B

3
m_C
1
g_A

3
m_C
4
g_D

4
m_D
1
g_A