即使

Question

我使用下面的查询的条目不存在返回值：

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行可能与shop_images没有联系到行存在。因此，... WHERE shop_entrys.id = shop_images.s_id ...将不会被满足。在这种情况下，我仍然想返回结果。例如：

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

我应该如何改变上述查询仍返回一个结果？

Answer 1

使用LEFT OUTER JOIN与合并成一个默认值，以与第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 

请避免使用隐式连接语法（逗号分隔），因为它已过时，杂乱，很多时候会导致错误。仅使用加入的正确语法！