题解 |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

WITH t1 AS (
SELECT
*
FROM answer_tb
LEFT JOIN issue_tb
    USING(issue_id))
    
SELECT
COUNT(DISTINCT a.author_id) AS num

FROM t1 AS a
JOIN t1 AS b
    ON a.author_id = b.author_id
    
WHERE a.issue_type = 'Career' AND b.issue_type = 'Education'