*[DB]230118_Wed

데이터의 집계 및 결합

 

DISTINCT : 중복된 데이터 제거

 - SELECT DISTINCT 컬럼 FROM 테이블

 

// join 과 union 의 차이? 

 

271p

select 
rating.user_id, rating.rating, 
ramen.name, ramen.quantity, ramen.is_spicy
from
rating left join ramen
on ramen.id = rating.item_id and rating.item_type = 'ramen';

272p

select 
rating.user_id, rating.rating, 
ramen.name, ramen.quantity, ramen.is_spicy
from
rating left join ramen
on ramen.id = rating.item_id and rating.item_type = 'ramen';

-- left:20 inner: 11, right :12  ,full : 21

 

274p 이중조인

select users.name, ramen.name, rating.rating, ramen.quantity, ramen.is_spicy
from
(rating join ramen
on ramen.id = rating.item_id and rating.item_type = 'ramen'
	)join users 
on users.id = rating.user_id
where rating >=3;

 

테이블 JOIN 테이블 ON 연결할 조건

테이블 JOIN 테이블 USING ( 연결할 컬럼 목록 )