如何限制不帶子選擇的從左連接返回的結果數量？

Question

我有以下查詢，但它從profile_photos表中返回多個條目。是否可以從個人資料照片表中只返回一個隨機條目？如果不是隨機的，可以將結果數量限制爲1？

這裏是我當前的查詢：

$sql = "SELECT * FROM login_users " . 
     "LEFT JOIN profile_photos ON profile_photos.user_id = login_users.user_id " . 
     "LEFT JOIN profiles ON profiles.user_id = login_users.user_id " . 
     "WHERE login_users.restricted <> 1 " . 
     "ORDER BY login_users.birthdate DESC " . 
     "LIMIT 0, 20"; 

Answer 1

這是一種方法。查找隨機的照片，然後在附加信息加入：

SELECT * 
FROM (select lu.*, 
      (select photo_id 
       from profile_photos 
       where lu.user_id = profile_photos.user_id 
       order by RAND() 
       limit 1 
      ) as photo_id 
     from login_users lu 
    ) lu LEFT JOIN 
    profile_photos pp 
    ON pp.photo_id = lu.photo_id LEFT JOIN 
    profiles p 
    ON p.user_id = lu.user_id 
WHERE lu.restricted <> 1 
ORDER BY lu.birthdate DESC 
LIMIT 0, 20 

這是假定photo_id是profile_photos主鍵。