题解 | 请写出计算粉丝ctr的sql语句

select
round(sum(read_num) / sum(show_num), 4) fans_ctr
from 
c
where (content_id, fans_id ) in (
                                select
                                content_id
                                ,fans_id
                                from
                                a
                                join
                                b
                                on a.author_id = b.author_id
                            )