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