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'
)