题解 | #某乎教育类问题的用户里有多少回答过职场类问题#

    SELECT COUNT(DISTINCT b.author_id) AS num
    FROM
            (SELECT s.author_id, s.issue_id, i.issue_type
            FROM answer_tb AS s LEFT JOIN issue_tb AS i
            ON s.issue_id= i.issue_id
            WHERE i.issue_type='Education') AS a
    LEFT JOIN 
            (SELECT s.author_id, s.issue_id, i.issue_type
            FROM answer_tb AS s LEFT JOIN issue_tb AS i
            ON s.issue_id= i.issue_id
            WHERE i.issue_type='Career') AS b
    ON  a.author_id=b.author_id
;