题解 | #某乎同时回答教育和职场类问题的用户个数#
某乎问答回答过教育类问题的用户里有多少用户回答过职场类问题
https://www.nowcoder.com/practice/b02cf9ee7b9f4cdda308f8155ff3415d
select
count(author_id) as num
from(
select
author_id
,count(edu.issue_type) as edu_num
,count(car.issue_type) as car_num
from answer_tb ans
left join
(select *
from issue_tb
where issue_type='Education') edu
using(issue_id)
left join
(select *
from issue_tb
where issue_type='Career') car
using(issue_id)
group by author_id
) t
where edu_num>0 and car_num>0
与多日留存的问题有点相似，利用两次left join issue_tb，通过NULL+count实现计算在某类问题上的回答次数
用left join的方式，会比in筛选运行速度快一些
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