SELECT level, 
    COUNT(DISTINCT uid) level_cnt
FROM exam_record a 
    JOIN user_info b USING(uid)
    JOIN examination_info c USING(exam_id)
WHERE c.tag = 'SQL' AND score > 80
GROUP BY level
ORDER BY level_cnt DESC, level DESC

承接上一题，根据exam_record表联结另外两个表，对应4,5行

限制条件对应6行

最后要输出等级和数量，并且首先按数量倒排，再按等级倒排，对应7,8行