题解 |某乎回答过教育类问题的用户有多少用户回答过职场类问题

select count(distinct author_id) as num
from (
    select a.author_id, a.issue_id, b.issue_type
    from answer_tb as a
    left join issue_tb as b
    on a.issue_id = b.issue_id
) as d
where author_id in (
    select distinct author_id
    from (
        select a.author_id, a.issue_id, b.issue_type
        from answer_tb as a
        left join issue_tb as b
        on a.issue_id = b.issue_id
    ) as c
    where issue_type = 'Education'
) and issue_type = 'Career';