题解 | #大小写混乱时的筛选统计#

select t3.tag,t4.answer_cnt
from(
select tag, count(start_time) answer_cnt
from examination_info t1
left join exam_record t2
on t1.exam_id = t2.exam_id
group by tag
having answer_cnt < 3) t3
left join 
(
    select tag,count(start_time) answer_cnt
    from examination_info t1
left join exam_record t2
on t1.exam_id = t2.exam_id
group by tag
) t4
on upper(t3.tag) = t4.tag
where t3.answer_cnt!= t4.answer_cnt
and t4.answer_cnt >0

题目描述的比较乱，建议一步一步来，先挑选出小于3的，再将其和大写的表进行链接。