Question

我有这样的查询

SELECT cus.id, cus.crawler_url_id, cus.created_at, cus.status
FROM crawler_url_stats cus
INNER JOIN (
    SELECT id, crawler_url_id, max( created_at ) latest
    FROM crawler_url_stats
    GROUP BY crawler_url_id
) cus2 ON ( cus.crawler_url_id = cus2.crawler_url_id
AND cus.created_at = cus2.latest )
ORDER BY `cus`.`crawler_url_id` ASC

这是在其他一个主题中提出的一个小修改版本的查询，我刚刚将内部查询中的GROUP BY更改为crawler_url_id（它不是id）并更改了匹配也加入crawler_url_id。

所有内容似乎都运行良好，但是created_at之一的行crawler_url_id为空时出现问题，例如我有4行crawler_url_id等于{{ 1}}如果只有其中一个5等于created_at，则结果不会包含null 5的结果。

对我来说似乎很奇怪，首先我尝试了这篇帖子https://stackoverflow.com/a/7745635/1951693中的代码，但它返回了错误的结果（每个crawler_url_id多个结果）所以我修改了它，现在它只是在那里挣扎在那里是空的。

Answer 1

尝试从子查询中排除NULL行：

SELECT cus.id, cus.crawler_url_id, cus.created_at, cus.status
FROM crawler_url_stats cus
INNER JOIN (
    SELECT id, crawler_url_id, max( created_at ) latest
    FROM crawler_url_stats
    WHERE created_at IS NOT NULL
    GROUP BY crawler_url_id
) cus2 ON ( cus.crawler_url_id = cus2.crawler_url_id
AND cus.created_at = cus2.latest )
ORDER BY `cus`.`crawler_url_id` ASC

Answer 2

在两个子查询中设置max(ifnull(created_at, 0))而不是cus.created_at，效果很好。解决。

如果column为null，则选择Max（）列不会返回

2 个答案: