分组查询<Group by子句>
select 字段名列表 from 表名 [where 条件表达式] group by 字段名
select 字段名列表 from 表名 [where 条件表达式] group by 字段名 Having <筛选表达式>
select cno,avg(degree) as chengji from score group by cno
select cno,avg(degree) as chengji from score group by cno Having avg(degree)>80