当内部 mysql case 语句未显示在结果中时不匹配

Question

我有一个mysql语句如下

SELECT CASE
       WHEN HOUR(created_at) BETWEEN 0 AND 11 THEN 'Morning'
       WHEN HOUR(created_at) BETWEEN 12 AND 15 THEN 'Afternoon'
       WHEN HOUR(created_at) BETWEEN 16 AND 18 THEN 'Evening'
       WHEN HOUR(created_at) BETWEEN 19 AND 24 THEN 'Night'
   END AS session,
   SUM(total) AS `total` FROM `orders`  WHERE (purchase_date between '2014-05-01' and '2014-05-30')
   GROUP BY CASE
         WHEN HOUR(created_at) BETWEEN 0 AND 11 THEN 1
         WHEN HOUR(created_at) BETWEEN 12 AND 16 THEN 2
         WHEN HOUR(created_at) BETWEEN 17 AND 18 THEN 3
         WHEN HOUR(created_at) BETWEEN 19 AND 24 THEN 4
     END;

我得到这样的输出

+------------+------------+
| session    | total      |
+------------+------------+
| Morning    |   47083.21 |
| Afternoon  | 1124804.51 |
| Evening    |  165643.34 |
| Night      | 1690492.01 |
+------------+------------+

但是当早上没有条目时，输出会在结果中缺少 Morning 行，但我想要一个包含早晨但总计为 0 的行。 请帮助我如何实现同样的目标

Expected output

+------------+------------+
| session    | total      |
+------------+------------+
| Morning    |          0 |
| Afternoon  |   14804.51 |
| Evening    |   16643.34 |
| Night      |   19492.01 |
+------------+------------+

实际输出没有晨排

Actual output

+------------+------------+
| session    | total      |
+------------+------------+
| Afternoon  | 1124804.51 |
| Evening    |  165643.34 |
| Night      | 1690492.01 |
+------------+------------+

如果能提供任何帮助或提示来解决此问题，我将不胜感激

Answer 1

添加create table cal (hours int not null)。用 0 到 23 填充它（没有 24 小时？）。然后做

SELECT CASE
   WHEN cal.hours BETWEEN 0 AND 11 THEN 'Morning'
   WHEN cal.hours BETWEEN 12 AND 15 THEN 'Afternoon'
   WHEN cal.hours BETWEEN 16 AND 18 THEN 'Evening'
   WHEN cal.hours BETWEEN 19 AND 24 THEN 'Night'
END AS session
,sum(coalesce(total, 0)) from
(select  created_at, total from orders 
 where purchase_date between '2014-05-01' and '2014-05-30') T1
 right outer  join cal on (cal.hours = hour(T1.created_at))
group by CASE
   WHEN cal.hours BETWEEN 0 AND 11 THEN 'Morning'
   WHEN cal.hours BETWEEN 12 AND 15 THEN 'Afternoon'
   WHEN cal.hours BETWEEN 16 AND 18 THEN 'Evening'
   WHEN cal.hours BETWEEN 19 AND 24 THEN 'Night' END;