SQL聚合函数

count(*)总数

select count(*) from person;
select count(*) from person where age=18;

max最大值 min最小值

select max(age) from person;
select min(id) from person where age=17;

sum求和 avg平均值(默认4位小数)

select sum(age) from person where gender=1;
select avg(age) from person where gender=1;
select sum(age)/count(*)  from person where gender=1;

round四舍五入

select round(avg(age),2)  from person where gender=1;