mysql - SQL 返回 NULL 而不是空结果

Question

SQL FIDDLE HERE

我正在制作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空值，它会返回 NULL 或默认值(参见 SQL Fiddle)

我设法绕过了，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

你需要一个 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 中没有匹配项的行；因为您使用的是 LEFT JOIN 而不是 INNER JOIN，所以我假设您想要那些具有空匹配的行。

FIDDLE