题解 | SQL啰嗦了点，但是容易看懂

select
count(distinct ab.author_id)
from answer_tb ab left join issue_tb i 
on ab.issue_id=i.issue_id
where 
i.issue_type="Career"
and 
ab.author_id in (
select
ab.author_id
from answer_tb ab left join issue_tb i 
on ab.issue_id=i.issue_id
where
issue_type="Education"
)