先筛选出回答过Education的用户(子查询),在此基础上筛选回答过Career的用户

select count(distinct author_id) as num
from issue_tb t join answer_tb t1 using(issue_id)
where author_id in (select author_id from issue_tb t join answer_tb t1 using(issue_id) where issue_type = 'Education')
    and issue_type = 'Career'