group_by

1. In accordance with a column or columns of the plurality of data packets

2. polymerization operation for each group

3. judgment result of the polymerization

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;