SQL SELECT不同的外鍵

Question

我需要在兩個表中選擇外鍵彼此匹配，但是我不想重復相同的外鍵。

我在數據庫上有這些表：

|Photo| (0/*) --------- (1)|Advertisement|

    |Photo|
       -photo_id
       -url
       -advertisement_id

    |Advertisement|
       -advertisement_id
       ...

數據庫數據

|Photo|
       photo_id | url | advertisement_id
           1    |  x  |        1
           2    |  x  |        1
           3    |  x  |        2
           4    |  x  |        3

我要這個：

ID   | URL  
       1    |  x
       2    |  x    -> It is anyone url   
       3    |  x

我在下面嘗試SQL，但是此重復ID =（

SELECT a.advertisement_id as ID, p.url as URL  
    FROM Advertisement a  
    RIGHT JOIN Photo p
    ON a.advertisement_id = p.advertisement_id
    LIMIT 6

結果：

ID  |  URL  
    1   |   http://i0.statig.com.br/bancodeimagens/18/46/2h/18...
    2   |   http://www.yaves.es/images/Animales/Tiernos-Gatito...
    2   |   http://images.forwallpaper.com/files/thumbs/previe...
    10  |   http://www.petmag.com.br/img/gatos/racas/7892/euro...
    7   |   http://www.gmstatic.com/content/images/1369932859_...
    8   |   http://www.navegandonaweb.com/wp-content/uploads/2...

有人幫我構造SQL查詢嗎？

Answer 1

SELECT a.advertisement_id as ID, p.url as URL  
FROM Advertisement a  
RIGHT JOIN Photo p
ON a.advertisement_id = p.advertisement_id
GROUP BY a.advertisement_id
ORDER BY p.photo_id ASC (or DESC ? )
LIMIT 6

要么

SELECT a.advertisement_id as ID, p.url as URL  
FROM Advertisement a,Photo p
WHERE a.advertisement_id = p.advertisement_id
GROUP BY a.advertisement_id
ORDER BY p.photo_id ASC (or DESC ? )
LIMIT 6