题解 | #回答过教育类问题的用户多少用户回答过职场类问题#

select count(a.author_id) num
from
(
select distinct author_id
from answer_tb ans 
left join issue_tb iss on ans.issue_id = iss.issue_id
where issue_type = 'Education'
) a
inner join 
(
select distinct author_id
from answer_tb ans 
left join issue_tb iss on ans.issue_id = iss.issue_id
where issue_type = 'Career'
) b
on a.author_id = b.author_id

考点：纯粹考察表连接的题目

思路：取出回答过教育类题目的用户，再取出回答过职场类题目的用户，两者取交集，就是两种类型题目都回答过的用户。

注意📢需要用distinct对author进行去重