select
        sum(c.read_num) / sum(c.show_num) as fans_ctr
from (
    select
        b.content_id,
        a.fans_id
    from 
            b
    left join 
            a
    on 
            b.author_id = a.author_id
) as content_fans_table
inner join 
        c
on 
        content_fans_table.content_id = c.content_id
and
        content_fans_table.fans_id = c.fans_id