分组：GROUP BY

GROUP BY 必须配合聚合函数使用（COUNT/SUM/AVG/MAX/MIN）
SELECT 后面只能出现两种东西
分组字段（GROUP BY 里的字段）
聚合函数（COUNT/SUM...）

---

create table studenter
(

name nvarchar(10),
gender nvarchar(2),
score int

);

insert into studenter
values
('张三','男',80),
('李四','女',90),
('王五','男',70),
('赵六','女',85),
('钱七','男',60);

select
gender,
avg(score) as 平均分
from studenter
group by gender; --按照男女生分组

--分组+条件过滤 having

--where 分组前过滤 having 分组后过滤

select
gender,
count() AS 人数
from studenter
group by gender
having count() >=3;