题解 | #回答过教育问题的用户里有多少用户回答过职场问题#

SELECT COUNT(DISTINCT C.author_id) FROM 
(
     SELECT 
    B1.author_id
    FROM issue_tb A1
    LEFT JOIN answer_tb  B1
    ON A1.issue_id = B1.issue_id
    where issue_type IN( 'Education')
)C
INNER JOIN 
(
    SELECT 
    B2.author_id
    FROM issue_tb A2
    LEFT JOIN answer_tb  B2
    ON A2.issue_id = B2.issue_id
    WHERE A2.issue_type IN( 'Career')
)D
WHERE C.author_id = D.author_id

（1）统计回答过教育类问题的用户

• [条件]：where issue_type = 'Education'

（2）统计回答回答过职场类问题的用户

• [条件]：where issue_type = 'Career'

（3）取2段查询结果的交集，并进行去重计数

• COUNT(DISTINCT C.author_id)