MySQL COUNT return 0 如果它不为空

Question

我需要统计日期之间的所有文章。我使用 IFNULL(COUNT(*), 0) 其中 returns 为空。

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;

The result is:

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

The result I want its:

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

Answer 1

您需要在主查询而不是子查询中执行 IFNULL()。子查询没有为缺失的日期返回任何行，因此甚至没有要转换为 0 的空计数。

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;

Answer 2

您的查询似乎比需要的更复杂。我没有看到按日期分组 post 和在日期上过滤两次的子查询的意义。

我觉得你想要的逻辑是：

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率，尤其是因为未在 post 日期列上应用日期函数。我会推荐 posts(post_created_at) 上的索引以充分利用此代码。

MySQL COUNT return 0 如果它不为空

MySQL COUNT return 0 if it's not null

mysql

sql

count

aggregate-functions

date-arithmetic