两种方法：子查询和having
区别在于group搭配where和having的顺序问题。
where先执行，再groupby分组；groupby先分组，having再执行。
——搭配where，嵌入子查询

SELECT T.number from(
SELECT count(number) coun,number
from grade
group by number)as T
where T.coun>=3;

——搭配having

SELECT number
from grade
group by number
HAVING count(number)>=3;