题解 | #回答过类问题的用户里有多少用户回答过B类问题#

select
    count(distinct if(issue_type = 'Career',author_id,null)) as num
from 
    answer_tb as a
left join issue_tb as b
on a.issue_id = b.issue_id
where a.author_id in(
    select 
        author_id
    from 
        answer_tb as t1
    left join issue_tb as t2
    using(issue_id) 
    where 
        issue_type = 'Education'
)