with t1 as(
select author_id,issue_id from answer_tb
where issue_id in (select issue_id	from issue_tb
where issue_type = 'Education' )
),
t2 as (
select author_id,issue_id from answer_tb
where issue_id in (select issue_id	from issue_tb
where issue_type = 'Career' )
)
select count(distinct author_id) as num
from t1 join t2 using(author_id)

选择两个表 相互连接