题解 | 分组过滤练习题（方法2）

select
    university, a.avg_question_cnt, a.avg_answer_cnt  #引用子表字段名称的格式：子表名称.字段名称
from
    (
        select
            university,
            avg(question_cnt) as avg_question_cnt,
            avg(answer_cnt) as avg_answer_cnt
        from 
            user_profile group by university
    ) as a  #作为子表使用时记得对子表进行命名
where
    a.avg_question_cnt <5 or a.avg_answer_cnt < 20