MySQL查询多个表

Question

电贺！我有12张桌子，一年中每个月都有一张桌子：

January

+----+-----+  
| id | sale|  
+----+-----+  
|  1 | 250 |  
|  3 | 500 |
|  5 | 200 |  
|  7 | 100 |  
+----+-----+

February

+----+-----+  
| id | sale|  
+----+-----+  
|  1 | 350 |  
|  2 | 400 |
|  3 | 500 |  
|  4 | 800 |  
+----+-----+

等

我需要进行查询，结果如下：

Annual Sales
+----+-----------+-----------+
| id | Sales_Jan | Sales_Feb |
+----+-----------+-----------+
|  1 |       250 |       350 |
|  2 |         0 |       400 |
|  3 |       500 |       500 |
|  4 |         0 |       800 |
|  5 |       200 |         0 |
|  7 |       100 |         0 |
+----+-----------+-----------+

如果来自两个表的匹配ID不重复，则通过放置0或任何其他符号来显示来自其他月份的缺失ID，表明该ID没有任何销售月份。

非常感谢！

Answer 1

您可以使用union all和聚合：

来解决此问题

select id,
       sum(case when month = 'Jan' then sale else 0 end) as Jan_Sale,
       sum(case when month = 'Feb' then sale else 0 end) as Feb_Sale,
       . . .
       sum(case when month = 'Dec' then sale else 0 end) as Dec_Sale
from ((select 'Jan' as month, id, sale from January) union all
      (select 'Feb' as month, id, sale from February) union all
      . . .
      (select 'Dec' as month, id, sale from February)
     ) t
group by id;