쿼리에 여러 JOIN이 있습니다.

Question

나중에 사용할 수있는 연락처 테이블을 만들고 싶습니다. 그래서 구조 :

table contact: id, contact_type 
table contact_meta : id, contact_id, meta_key, meta_value 

이 쿼리는 나쁜 습관입니까? 쿼리가 아닌 "나쁜 관행"

select c.id, 
    cm1.meta_value as name, 
    cm2.meta_value as email, 
    cm3.meta_value as bussiness_phone 
    from contact as c 
    left join contact_meta as cm1 on (cm1.contact_id = c.id) 
    left join contact_meta as cm2 on (cm2.contact_id = c.id) 
    left join contact_meta as cm3 on (cm3.contact_id = c.id) 
    where c.contact_type = 'supplier' and 
    cm1.meta_key = 'name' and 
    cm2.meta_key = 'email' and 
    cm3.meta_key = 'bussiness_phone' ORDER BY c.id DESC 

Answer 1

을 (? 10 개 이상 가입),하지만 자신이 속한 조인 절에 추가 조건을 이동 :

select c.id, 
    cm1.meta_value as name, 
    cm2.meta_value as email, 
    cm3.meta_value as bussiness_phone 
from contact as c 
left join contact_meta as cm1 on cm1.contact_id = c.id and cm1.meta_key = 'name' 
left join contact_meta as cm2 on cm2.contact_id = c.id and cm2.meta_key = 'email' 
left join contact_meta as cm3 on cm3.contact_id = c.id and cm3.meta_key = 'bussiness_phone' 
order by c.id desc 

참고 불필요한 브라켓의 제거를 .