group_by

1.按照一个列或者多个列对数据分组

2.对每个组进行聚合操作

3. 对聚合后的结果进行判断

1. select avg(score) as score from teacher 

2. select  grade, avg(score) as avg_score from teacher group by grade

3. select  grade, avg(score) as avg_score from teacher group by grade having avg_score>30;