题解 | #大小写混乱时的筛选统计#

WITH t_tag_count as (
    SELECT tag, COUNT(uid) as answer_cnt
    FROM exam_record
    JOIN examination_info USING(exam_id)
    GROUP BY tag
)
select a.tag,b.answer_cnt  from t_tag_count as a, t_tag_count as b where 
upper(a.tag) = b.tag AND a.tag != b.tag AND a.answer_cnt < 3

本题只要通过自连接，将表a的tag upper后与b一致，求出a.tag和b.answer_cnt即可