SQL 常用聚合统计函数

-- 最大值：88
select max(math) as max_math from students
-- 最小值：54
select min(math) as min_math from students
-- 人员数量，就是数据的数量 9
select count(name) as count_name from students
-- 有时可以这么写 9，星号 null 值计入
select count(*) as count_data from students
-- 数字平均成绩 73.44444444444444
select avg(math) as avg_math from students
--数学成绩总和 661
select sum(math) as sum_math from students

-- 有几个班：3
select count(distinct class) as class_qty from students
-- 有几个性别：2
select count(distinct gender) as gender_qty from students