题解 | #某乎问答回答过教育类问题的用户里有多少用户回答过职场类问题#
某乎问答回答过教育类问题的用户里有多少用户回答过职场类问题
http://www.nowcoder.com/practice/b02cf9ee7b9f4cdda308f8155ff3415d
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