聚合函数:对行数据进行合并
sum,avg,count,max,min
一般是对数字类型的列进行操作
一条查询中可以同时写多个聚合函数,但是不能与普通列混写
聚合中的null问题:不参与计算
select count(*) from studentinfo
select sum(score)
from sc
inner join subjectinfo as su on sc.sid=su.suid
where su.sucourse='语文'
select avg(score)
from sc
inner join subjectinfo as su on sc.sid=su.suid
where su.sucourse='语文'
select max(score)
from sc
inner join subjectinfo as su on sc.sid=su.suid
where su.sucourse='语文'
select min(score)
from sc
inner join subjectinfo as su on sc.sid=su.suid
where su.sucourse='语文'