select
    sum(c.read_num) / sum(c.show_num) as fans_ctr
from a 
    left join b on a.author_id = b.author_id
    left join c on b.content_id = c.content_id
where a.fans_id = c.fans_id
#莫名其妙的题，有点模糊，能做，简单，不值得花时间