题解 | #回答过教育类问题的用户里有多少用户回答过职场类#

select 
    count(distinct t1.author_id) num
from (
    select
        author_id
    from answer_tb
    where substr(issue_id,1,1) = 'E'
) t1
inner join (
    select
        author_id
    from answer_tb
    where substr(issue_id,1,1) = 'C'
) t2
on t1.author_id = t2.author_id