mysql - 在 MySQL 中加入两个表并按行 ID 添加值

Question

给出了两个具有共同 id 和整数值的表：

table1:            table2:
| id   | val  |    | id   | val  |
+------+------+    +------+------+
| 1    |   0  |    | 1    |   1  |
| 2    |   1  |    | 1    |   1  |
| 3    |   1  |    |  2   |   2  |
| 4    |   2  |    |  2   |   2  |
| 5    |   2  |    |   3  |   2  |
                   |   3  |   2  |
                   |   3  |   2  |
                   |    4 |   6  |  <id shifted for readability>

如何在一个查询中归档此结果：

result-table:
| id   | val  |
+------+------+
| 1    |   2  |
| 2    |   5  |
| 3    |   7  |
| 4    |   8  |
| 5    |   2  |

编辑：

按照我得到的一些答案：

SELECT t1.id, t1.val + SUM(t2.val) AS val
FROM table1 t1
LEFT JOIN table2 t2 ON (t1.id = t2.id)
GROUP BY t1.id

什么给了我：

id  val
1   2
2   5
3   7
4   8
5   NULL

但又2 plus nothing = 2不是NULL。有不同的方法吗？

Answer 1

你忘了group by：

SELECT t1.id, t1.val - SUM(t2.val) AS val
FROM t1
JOIN table2 t2 ON (t1.id = t2.id)
GROUP BY t1.id

当然，我不确定您为什么要尝试减去示例查询中的值。也许你的意思是+？

Answer 2

使用联合而不是加入：

     select id, sum(val)
     (
       SELECT t1.id, t1.val  as val 
       FROM table1 t1 
       union all # note this change to include all the value from 2 table  
       select t2.id, t2.val as val 
       from  table2 t2
      ) as unionTable
     groupby id;