指定变量值，有多少个这样的行

select COUNT(*) as count1 from student --as为列命名为count1
where cid=1

最大年纪值 / R语言为小写

select MAX(age) from student

白虎班的年龄平均，用到内连接

select AVG(age)
from class
inner join student on class.cid=student.cid

over函数和聚合函数结合，如果不用over，则会报错

select student.*,avg(age) over()
from student
where cid=2

统计1，2班人数,分组后信息被屏蔽，只能拿到分组那个数值和聚合函数的值

select cid, Count(*)
from student
group by cid

分组求年龄平均值

select cid,avg(age) from student
group by cid

每个班的男女生人数

select gender,cid,count(*)
from student
group by gender,cid
select * from student

统计学生编号大于2的各班级的各性别的学生人数

select gender,cid,count(*)
from student
where id>2
group by gender,cid

统计学生编号大于2的各班级的各性别的学生大于1的人数

select gender,cid,count(*)
from student
where id>2
group by gender,cid having count(*)>1 --having表示分组之后再做运算