select distinct uid, 'activity2' as activity from examination_info join exam_record using(exam_id) # 内连接两个表 where year(start_time)='2021' and difficulty='hard' and score>80 and timestampdiff(minute,start_time,submit_time)<30 # 选出第二种情况2021年完成困难题目，分数大于80，且用时小雨30分钟 union all # 合并第二种情况 select dis...
