题解 | #分组过滤练习题#

本题核心是掌握group by，group by 代表分组查询（此时可以引用表的别名），having是分组后的筛选。avg（）是聚合函数，依赖于group by。
执行循序为：from-》group by-》having-》select

select university,avg(question_cnt) as avg_question_cnt,avg(answer_cnt) as avg_answer_cnt
from user_profile group by university HAVING avg_question_cnt < 5 or avg_answer_cnt
< 20;