select * from (
select exam_id as tid,count(distinct uid) as uv,
count(start_time) as pv from 
exam_record group by exam_id order by uv desc ,pv desc) t1 
UNION 
select * from (
select question_id,count(distinct uid) as uv ,
count(submit_time) as pv 
from practice_record group by question_id 
order by uv desc,pv desc) t2