[데이터베이스 SELECT] 4. 복수행 함수

1. 세로 연산의 원리

2. 그룹함수

전체행을 세로연산 하려면 무엇을 써? - sum, max, min, count, avg(그룹함수)

• 평균

select avg(height)
from student;

• 갯수 (몇 개 있는지)

select count(height)
from student;

• 더하기

select sum(height)
from student;

• 최소값

select min(height)
from student;

• 최대값

select max(height)
from student;

특정 행동, 그룹을 세로연산 하려면 무엇을 써? - where과 그룹함수

select avg(weight)
from student
where year(birthday) = 1975;

select floor(avg(pay))
from professor
where position = "정교수";

select avg(height)
from student;

그룹별로 세로연산 하려면 무엇을 써? - group by 그룹함수

select avg(sal), deptno -- 동일하면 출력가능
from emp
where deptno  = "20";

select avg(sal), deptno 
from emp
where deptno  = 10
union all
select avg(sal), deptno 
where deptno  = 20
union all
select avg(sal), deptno
from emp
where deptno  = 30;

select avg(sal)
from emp
group by deptno;

그룹핑 원리 실습

select *
from student;

select avg(height), grade
from student
group by grade;

select job, deptno, avg(sal)
from emp
group by job, deptno; 
having deptno !=10 ;