问高效地在数据库中添加好友逻辑

EN

CREATE TABLE `friends` (
`user_id` INT(11) NOT NULL,
`friend_id` INT(11) NOT NULL,
PRIMARY KEY (`user_id`, `friend_id`)
)
ENGINE = INNODB;

INSERT INTO `friends` (`user_id`, `friend_id`)
VALUES
(1, 2), (1, 3), (1, 4), (2, 1), (2, 10), (4, 1), (4, 20);

SELECT users.*
FROM users
INNER JOIN friends ON users.user_id = friends.friend_id 
WHERE friends.user_id = [The ID of current user]

friends (user_id, user_id)