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

select 
    count(distinct case when issue_type = 'Career' then author_id else null end) as num
from 
   answer_tb a left join  issue_tb i using(issue_id)
where
    author_id in (
        select distinct author_id
        from
            answer_tb a left join  issue_tb i using(issue_id)
        where 
            issue_type = 'Education'
    )