sql group by season

Question

我在使用SQL中的'group by'和'select-case-when'时遇到问题。

我有这张表：Promotion（Name，CodProd，CodProdSimilar，StartDate，EndDate，Discount）以及我需要做的是：了解哪些季节是促销活动。

例如：

<table style="width:100%"> 
    <tr> 
    <td>Name</td> 
    <td>StartDate(dd/mm/yyyy)</td> 
    <td>EndDate(dd/mm/yyyy)</td> 
    <td>Season (southern hemisphere)</td> 
    </tr> 
    <tr> 
    <td>Prom1 </td> 
    <td>02/01/2015 </td> 
    <td>09/01/2015</td> 
    <td>Summer</td> 
    </tr> 
    <tr> 
    <td>Prom2 </td> 
    <td>02/01/2015 </td> 
    <td>09/04/2015</td> 
    <td>Summer</td> 
    </tr> 
    <tr> 
    <td>Prom2 </td> 
    <td>02/01/2015 </td> 
    <td>09/04/2015</td> 
    <td>Autumn</td> 
    </tr> 
</table> 

这是很容易解决的时候促销是唯一一个赛季，但我不能为Prom2的情况下做到这一点呢。

我的代码：

select 
    Name, StartDate, EndDate, 
    CASE 
     WHEN EXTRACT(MONTH from StartDate) < 3 THEN 'Summer' 
     WHEN EXTRACT(MONTH from StartDate) = 3 THEN 
      CASE 
       WHEN EXTRACT(DAY from StartDate) <= 20 THEN 'Summer' 
       ELSE 'Autumn' 
      END 
     WHEN EXTRACT(MONTH from StartDate) < 6 THEN 'Autumn' 
     WHEN EXTRACT(MONTH from StartDate) = 6 THEN 
      CASE 
       WHEN EXTRACT(DAY from StartDate) <= 20 THEN 'Autumn' 
       ELSE 'Winter' 
      END 
     WHEN EXTRACT(MONTH from StartDate) < 9 THEN 'Winter' 
     WHEN EXTRACT(MONTH from StartDate) = 9 THEN 
      CASE 
       WHEN EXTRACT(DAY from StartDate) <= 20 THEN 'Winter' 
       ELSE 'Spring' 
      END 
     WHEN EXTRACT(MONTH from StartDate)< 12 THEN 'Spring' 
     WHEN EXTRACT(MONTH from StartDate) = 12 THEN 
      CASE 
       WHEN EXTRACT(DAY from StartDate) <= 20 THEN 'Spring' 
       ELSE 'Summer' 
      END 
    END as season 
from 
    promotion 

任何想法？

非常感谢您的时间！

Answer 1

对于您的情况，您可以将列转换为字符串，使用月份&日期我们可以确定季节，如您所述。

select Name,StartDate,EndDate, 
case 
      when to_char(StartDate,'MMDD') between '0321' and '0620' then 'spring' 
      when to_char(StartDate,'MMDD') between '0621' and '0922' then 'summer' 
      when to_char(StartDate,'MMDD') between '0923' and '1220' then 'fall' 
      else              'winter' 
     end as season 
FROM promotion 

请参考这一点，你会得到更多的信息

https://wiki.postgresql.org/wiki/Date_and_Time_dimensions

PostgreSQL group by season (northern and southern hemisphere)