Question

我正在创建一个简单的博客系统，其中帖子可以有多个类别，所以我有这个查询：

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返回任何内容它返回NULL或默认值（请参阅SQL小提琴）

我设法绕过但我想知道是否有人有更好的解决方案：

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，您将在dev_items_to_categories中过滤掉没有匹配项的行;由于您使用LEFT JOIN而不是INNER JOIN，我假设您希望这些行具有空匹配。

FIDDLE

Answer 2

你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

SQL返回NULL而不是空结果

2 个答案: