select count(c.author_id) as num from answer_tb c left join issue_tb d using(issue_id) where c.author_id in( select a.author_id from answer_tb a left join issue_tb b using(issue_id) where b.issue_type='Education') and d.issue_type='Career' 先查回答教育问题的ID,最后在查这些ID里回答职场问题的