MySQL COUNT 如果不是 null，则返回 0

Question

我需要计算日期之间的所有文章。 我使用了返回 null 的IFNULL(COUNT(*), 0) 。

SELECT posts_count, dates.fulldate
FROM dates
LEFT JOIN 
   (SELECT IFNULL(COUNT(post_id), 0) AS posts_count, DATE_FORMAT(post_created_at, "%Y-%m-%d") AS pdate
   FROM posts
   WHERE DATE_FORMAT(post_created_at, "%Y-%m-%d") BETWEEN NOW() - INTERVAL 31 DAY AND NOW()
   GROUP BY DATE_FORMAT(post_created_at, "%Y-%m-%d")) t
ON t.pdate = dates.fulldate
WHERE dates.fulldate BETWEEN CURDATE() - INTERVAL 31 DAY AND CURDATE() ORDER BY dates.fulldate ASC;

结果是：

NULL |  2020-05-20    
NULL |  2020-05-21    
NULL |  2020-05-22    
7634 |  2020-05-23    
51224 | 2020-05-24

我想要的结果：

   0 |  2020-06-16   
1233 |  2020-06-15    
4354 |  2020-06-14   
   0 |  2020-06-13    
   0 |  2020-06-12

Answer 1

您的查询似乎比它需要的更复杂。 我看不到按日期对帖子进行分组并在日期上过滤两次的子查询的意义。

我认为你想要的逻辑是：

select count(p.post_id) post_count, d.fulldate
from dates d
left join posts p
    on  p.post_created_at >= d.fulldate
    and p.post_created_at <  d.fulldate + interval 1 day
where d.fulldate >= current_date - interval 1 month
group by d.fulldate
order by d.fulldate

我希望这应该比您的原始代码更有效，特别是因为没有在发布日期列上应用日期 function。 我建议在posts(post_created_at)上建立一个索引，以充分利用这段代码。

Answer 2

您需要在主查询中执行IFNULL() ，而不是子查询。 子查询没有返回缺少日期的任何行，因此甚至没有 null 计数可以转换为 0。

SELECT IFNULL(posts_count, 0) AS posts_count, dates.fulldate
FROM dates
LEFT JOIN 
   (SELECT COUNT(*) AS posts_count, DATE_FORMAT(post_created_at, "%Y-%m-%d") AS pdate
   FROM posts
   WHERE DATE_FORMAT(post_created_at, "%Y-%m-%d") BETWEEN NOW() - INTERVAL 31 DAY AND NOW()
   GROUP BY DATE_FORMAT(post_created_at, "%Y-%m-%d")) t
ON t.pdate = dates.fulldate
WHERE dates.fulldate BETWEEN CURDATE() - INTERVAL 31 DAY AND CURDATE() 
ORDER BY dates.fulldate ASC;