聚合函数

select MAX(age)  from Students --取出年龄最大的值（单个值）
select MIN (age ) from Students --取出年龄最小的值
select AVG (age) from Students --取出平均值
select MAX (age),MIN (age),avg(Age)from Students --取出最大和最小和平均值
select COUNT (Name) from Students --查询一个有几条满足条件的个数
select COUNT (name) from Students where Age >=3 --取出大于等于三的总个数
--聚合函数对null值不进行统计
select MAX (name),MIN (name)from Students