SQL返回NULL而不是空结果

Question

SQL此处

我正在创建一个简单的博客系统，其中帖子可以具有多个类别，因此我有以下查询：

SELECT *, GROUP_CONCAT(item_category) AS item_categories
FROM (`dev_pages`)
LEFT JOIN `dev_items_to_categories` ON `dev_items_to_categories`.`item_id` = `dev_pages`.`page_id`
WHERE deleted_time IS NULL
AND `page_type` =  'blog'
AND `item_category` =  '16'
ORDER BY `page_title` ASC

它工作正常，但如果没有结果而不是没有返回结果，则返回NULL或默认值（请参见SQL Fiddle）

我设法解决了这个问题，但我想知道是否有人对此有更好的解决方案：

SELECT a.* FROM (

    SELECT *, GROUP_CONCAT(item_category) AS item_categories
    FROM (`dev_pages`)
    LEFT JOIN `dev_items_to_categories` ON `dev_items_to_categories`.`item_id` = `dev_pages`.`page_id`
    WHERE deleted_time IS NULL
    AND `page_type` =  'blog'
    AND `item_category` =  '16'
    ORDER BY `page_title` ASC

) AS a
WHERE page_id > 0 

Answer 1

您需要一个GROUP BY子句：

SELECT *, GROUP_CONCAT(item_category) AS item_categories
FROM (`dev_pages`)
LEFT JOIN `dev_items_to_categories` ON `dev_items_to_categories`.`item_id` = `dev_pages`.`page_id` AND `item_category` =  '16'
WHERE deleted_time IS NULL
AND `page_type` =  'blog'
GROUP BY dev_pages.page_id
ORDER BY `page_title` ASC

您还需要将item_category = '16'放入ON子句中。 否则，您将过滤掉dev_items_to_categories没有匹配项的dev_items_to_categories ； 因为您使用的是LEFT JOIN而不是INNER JOIN ，所以我假设您希望这些行具有空匹配项。

小提琴

Answer 2

您也可以尝试合并功能。

SELECT *, GROUP_CONCAT(coalece(item_category,'')) AS item_categories
FROM (`dev_pages`)
LEFT JOIN `dev_items_to_categories` ON `dev_items_to_categories`.`item_id` = `dev_pages`.`page_id`
WHERE deleted_time IS NULL
AND `page_type` =  'blog'
AND `item_category` =  '16'
ORDER BY `page_title` ASC