--聚合函数

       -- sum()

       -- 求和

       select sum(age) from student;

       -- count()

       -- 求数量

       -- 数据量

       select count(*) as '数量' from student;

       -- max()

       --最大值

       select max(age) as '最大值' from student;

       -- min

       -- 最小值

       select min(age) as '最小值' from student;

       -- avg()

       --求平均值

       select avg(age) as '平均值' from student;

       --round()

       --保留几位小数

       select round(avg(age),2) as '平均值2位小数' from student;

       -- 综合查询

       select sum(age)/count(age) as '平均值' from student;