CREATE TABLE Users (
    id_user  INTEGER PRIMARY KEY,
    name     VARCHAR,
    email    VARCHAR UNIQUE,
    password VARCHAR
);

CREATE TABLE Friends (
    id_friends     INTEGER PRIMARY KEY,
    id_user        INTEGER REFERENCES Users (id_user),
    id_user_friend INTEGER REFERENCES Users (id_user) 
);

SELECT 
  u2.*
FROM
  `Users` u
  JOIN Friends f USING(`id_user`)
  JOIN `Users` u2 ON f.`id_user_friend` = u2.`id_user`
WHERE
  u.`email` = 'email@mail.ru';