即使

Question

我使用下面的查詢的條目不存在返回值：

SELECT shop_entrys.id, shop_images.path FROM shop_entrys,shop_images 
WHERE shop_entrys.id = shop_images.s_id AND 
shop_images.pos = 0 AND 
shop_entrys.category_id = 1 
ORDER BY shop_entrys.pos ASC 

然而，在shop_entrys行可能與shop_images沒有聯繫到行存在。因此，... WHERE shop_entrys.id = shop_images.s_id ...將不會被滿足。在這種情況下，我仍然想返回結果。例如：

shop_entrys.id shop_images.path 1 "/img1.jpg" ... ... 42 "not found"

我應該如何改變上述查詢仍返回一個結果？

Answer 1

使用LEFT OUTER JOIN與合併成一個默認值，以與第2列

SELECT shop_entrys.id, COALESCE(shop_images.path, 'NOT FOUND') 
FROM shop_entrys 
LEFT OUTER JOIN shop_images 
ON shop_entrys.id = shop_images.s_id AND shop_images.pos = 0 
WHERE shop_entrys.category_id = 1 
ORDER BY shop_entrys.pos ASC 

Answer 2

使用LEFT JOIN和COALESCE()：

SELECT shop_entrys.id, 
     COALESCE(shop_images.path,'NOT FOUND') 
FROM shop_entrys 
LEFT JOIN shop_images 
ON(shop_entrys.id = shop_images.s_id AND 
    shop_images.pos = 0) 
WHERE shop_entrys.category_id = 1 
ORDER BY shop_entrys.pos 

請避免使用隱式連接語法（逗號分隔），因爲它已過時，雜亂，很多時候會導致錯誤。僅使用加入的正確語法！