题解 | #网易云音乐推荐(网易校招笔试真题)#

梳理一下查找数据的逻辑：

1、找出关注者user_id等于1（以下统称关注者），其对应的被关注者id（即follower_id，以下统称被关注者）

select
    follower_id
from follow 
where user_id = 1

2、找出被关注者喜欢的音乐id

select
    music_id
from music_likes as ml  
join (
    select
        follower_id
    from follow 
    where user_id = 1 
     ) as t1
on ml.user_id = t1.follower_id

3、找出关注者已喜欢的音乐

select
    music_id
from music_likes
where user_id = 1

4、找出被关注者喜欢但排除关注者喜欢的音乐的名字

select
    music_name as name 
from music
where id in (
    select
        music_id
    from music_likes as ml  
    join (
        select
            follower_id
        from follow 
        where user_id = 1 
         ) as t1
    on ml.user_id = t1.follower_id
    where music_id not in (
        select
            music_id
        from music_likes
        where user_id = 1
        )
            )