sql复习-聚合函数

--SQL聚合函数--
count():求满足条件的记录数，与值无关
max():求最大值
min():求最小值
sum():求和  只能对数值进行计算不能运用于日期和字符串
avg():求平均值
--1.求整个表的记录数,随意传入参数,因为它与具体的值无关,只与记录数有关
select COUNT(sex) from Student
--查询班级年龄最小的学员和最大的学员
select MAX(BornDate) from Student
select min(BornDate) from Student
--------不管什么类型都可以使用max/min进行计算，如果是数值就比较大小，如果是字符串变比较拼音的大小--------------
select MAX(sex) from Student --nv
select min(sex) from Student--nan
--查询成绩最高分和最低分,平均分和总分
select MAX(StudentResult)  from Result
select MIN(StudentResult)  from Result

select sum(StudentResult)  from Result where StudentNo=2
select avg(StudentResult)  from Result where StudentNo=2
---不能使用sum/avg去计算日期类型的值
select sum(BornDate)  from Student where StudentNo=2
select avg(BornDate)  from Student where StudentNo=2
--也不能运用于字符串类型
select sum(StudentName)  from Student where StudentNo=2
select avg(StudentName)  from Student where StudentNo=2