Получение всех фильмов:
SELECT *
FROM FILMS
Получение всех пользователей:
SELECT *
FROM USERS
Получение топ 50 фильмов:
SELECT F.* FROM
(SELECT FILM_ID
FROM LIKES
GROUP BY FILM_ID
ORDER BY COUNT(USER_ID) DESC
LIMIT ?) AS L
INNER JOIN FILMS AS F ON L.FILM_ID=F.FILM_ID
Получение списка общих друзей пользователя 1 с пользователем 2:
SELECT * FROM USERS WHERE USER_ID IN
(SELECT U.FRIEND_ID
FROM (SELECT *
FROM FRIENDS
WHERE USER_ID = 1) AS U
INNER JOIN FRIENDS AS F ON F.FRIEND_ID = U.FRIEND_ID
WHERE F.USER_ID = 2)