MySql查詢與反向連接

Question

我想建立一個查詢返回與特定機會相關的聯繫人列表。

我有3個表：聯繫人，商機和關係（多對多）

接觸

id name 
--------- 
1 Davey Jones 
2 Bob Hope 
3 Tiger Woods 
4 Hillary Clinton 

機會

id description 
------------------ 
1  visit the locker 
2  singing and dancing 
3  playing golf 
4  laughing and crying 

關係

id  firstid  firsttype  secondid  secondtype 
--------------------------------------------------------- 
1  1   contact  1   opportunity 
2  3   opportunity 3   contact 
3  4   contact  4   opportunity 
4  4   opportunity 3   contact 

現在，如果我有opportunity_id，我想返回與該機會相關聯的所有聯繫人。

所以，如果opportunity_id = 4，成功查詢的結果應該是：

Hillary CLinton 
Tiger Woods 

但是，這是我的查詢，只返回1個記錄：

SELECT 
contacts.name 
FROM 
contacts 
INNER JOIN relationships ON contacts.id = relationships.secondid 
INNER JOIN opportunities ON opportunities.id = relationships.firstid 
where 
opportunities.id=4 
and (relationships.firsttype='opportunity' and relationships.secondtype='contact') 
or (relationships.firsttype='contact' and relationships.secondtype='opportunity') 

我停留在如何在這個查詢中做翻轉連接。

編輯：我剛剛發現UNION，然後嘗試這樣做，它似乎工作：

(select contacts.name from contacts where contacts.id = 
    (select secondid as id from relationships where (firstid = 4 and (firsttype='opportunity' and secondTtpe='contact')))) 
    UNION 
(select contacts.name from contacts where contacts.id = 
    (select firstid as id from relationships where (secondid = 4 and (firsttype='contact' and secondtype='opportunity')))) 

但這似乎笨重。這是處理這個問題的最好方法嗎？

Answer 1

在一個簡單的方法，你需要改變你的表relationships的設計，我會建議：

關係

id  id_contact  id_opportunity 
------------------------------------- 
1  1    1 
2  3    3 
3  4    4 
4  3    4 

與 relationships變化

所以，你可以查詢一下像：

SELECT contacts.name 
FROM contacts 
INNER JOIN relationships ON contacts.id = relationships.id_contact 
INNER JOIN opportunities ON opportunities.id = relationships.id_opportunity 
WHERE opportunities.id=4; 

你必須記住規範化你的表（所以表relationships必須有其他表中的標識符，只需要一個來自每個表...這就是爲什麼是一個標識符），並且在many-to-many表中更加小心。

Answer 2

試試這個：

SELECT contacts.name FROM contacts 
    inner join (
     SELECT * from relationships 
     where 
      (firstid = 4 and (firsttype='opportunity' and secondtype='contact')) 
      or 
      (secondid= 4 and (firsttype='contact' and secondtype='opportunity')) 
    ) rel 
    on contacts.id = rel.id;