使用in运算符分步查询

1. 在examination_info表中找出SQL试卷的exam_id
2. 在exam_record表中根据找到的exam_id找出做了SQL试卷且分数大于80的uid
3. 在user_info表中根据uid找到相应的用户，使用相关聚合函数完成查询

select 
	level,count(uid) level_cnt 
from 
	user_info 
where 
	uid in
		(select uid from exam_record where exam_id in --第二步
			(select exam_id from examination_info where tag='SQL') and score>80) --第一步
group by 
	level
order by 
	level_cnt desc  --第三步