WITH t_tag_count as (
    SELECT tag, COUNT(uid) as answer_cnt
    FROM exam_record
    LEFT JOIN examination_info USING(exam_id)
    GROUP BY tag
)

select a.tag,b.answer_cnt
from t_tag_count as a
join
t_tag_count as b

on upper(a.tag) = b.tag and a.tag != b.tag and a.answer_cnt < 3

自连接的应用，先找到所有答题标签和次数，然后自连接两个表，按照要求找目标答案。