多對多查詢MySQL

Question

我有兩個表：用戶[id，email]和角色[id，name]與多對多連接表user_has_roles [user_id，role_id]。 我試圖讓這顯示哪些用戶具有特定角色表，如果沒有的話空（或0）類似如下：

  | role 1 | role 2 | role 3 
--------------------------------------- 
user1@email | 1  | null | 1 
user2@email | null | null | 1 
--------------------------------------- 

感激有這方面的幫助。

Answer 1

select u.email, 
     case when sum(r.name = 'role 1') > 0 then 1 else null end as `role 1`, 
     case when sum(r.name = 'role 2') > 0 then 1 else null end as `role 2`, 
     case when sum(r.name = 'role 3') > 0 then 1 else null end as `role 3` 
from users u 
left join user_has_roles ur on ur.user_id = u.id 
left join roles r on ur.role_id = r.id 
group by u.email